UNSC Southern Cross

"If there's one thing I know, it is that the Endo is a survivor. She has endured the worst the Covenant can throw at her, and while it may have taken some time, she eventually came back asking for more. And now that she has been rebuilt from bow to stern, I expect that spirit to live on for decades more; and when she finally meets the end, she, nor her crew, will not go quietly."

- Commander Tom Žagar, at the Endeavour 's recommissioning ceremony.

The UNSC Endeavour is a venerable Beagle-class assault destroyer, and one of two ships to have participated in the Harvest Campaign and the Siege of Sol.

Specifications
Like all Beagle-class destroyers, the UNSC Endeavour shares most of its hull and superstructure design with the Halberd-class destroyer. At 480 meters in length, it was only five meters shorter than the design it was based on. Past its arrowhead bow, the differences are minimal. This mainly involved replacing certain areas of the hull with 400 millimeters of grapheel armor, which while cheaper, was not as strong nor as light as titanium-A1. The bow is slightly different, with a larger hangar bay that is half-again as large as on its more common counterparts, and a smaller upper hull. The hangar is divided into two sections; a long flight deck that is located on the upper floor, and a general-purpose bay that accommodated both dropships and ground vehicles on the lower one. Both decks use different airlocks, so they can launch and receive different squadrons at the same time. However, only aircraft entering the front hangar door can benefit from the advanced gravitic breaking system. The upper plate on the bow that ran between the raised plates of titanium-A battleplate is unusual in that had a series of automated elevators and armatures that could rearm and refuel up to three space fighters in the middle of battle. Intended as a forward-thinking feature, it was hoped that the UNSC would introduce a smaller interceptor model to take advantage of it. No such aircraft would ever reach the light of day, so the system lies unused. In homage to its maritime namesakes, the Endeavour is often seen sporting red-or-blue war paint resembling waves on its bow and wings.

Armament
The UNSC Endeavour 's primary ship-to-ship armament consisted of a fixed 27F11B6 MAC battery, which was selected over heavier gun systems because of its smaller, lighter construction. Rather than focusing on raw firepower, this battery relied on smaller shells which did not require as much electricity to accelerate to high speeds. This meant it was quick to charge and fire, and also gave the Endeavour a further reach to hit opponents; this emphasis was felt to be more suitable against the small, nimble opponents that it was likely to face. It was made up of two coilguns of different calibers; a 647mm barrel and a smaller 400mm model. The larger barrel is only used for short-ranged skirmishes, as it could only fire unguided kinetic kill vehicles at slower velocities - issues which did not apply to its smaller partner.

Backing this up was a formidable network of secondary weapons, mixed between missiles and naval autocannons. This tier not only acted as the destroyer's lighter anti-ship component, but it also fulfilled its need for a heavy air defense suite as well. The most numerous of these was its 18 pods of M58 Archer missile pods, although the exact number would gradually increase over its lifespan. These were general-purpose missiles that could be fitted with a wide variety of warheads to drastically change their purpose. For example, those fitted with M58E decoy warheads were used to draw fire away from enemy point defenses. Each pod had five silos, allowing the Endeavour to fire off a salvo of 90 missiles at once. However, despite their numbers, these were easily defeated by Covenant pulse lasers. Eight turrets of twin M66 Sentry 279mm/36 cal naval autocannons were also installed, each installed on high-elevation Mark XIV mounts. These were fairly adaptable weapons on a warship, able to dynamically switch targets between enemy warships, incoming space fighters, and engage in planetary bombardment. They were particularly effective at overwhelming energy shields, especially when they concentrate their fire in a small region.

To protect itself from missiles and enemy fighters, the UNSC Endeavour boasted a mixed armament of point defense guns. These were the last line of defense, when enemy fighters had penetrated its countermeasures and anticraft missiles and impact was now assured; as such, they were all very fast to fire and automated to ensure accuracy. Originally, this complement was quite modest, with three quad M870 Rampart 50mm/50 cal cannons and 19 M710 Bulwark 25mm/60 cal guns there to answer its needs. The Ramparts were positioned in key positions to provide an all-encompassing screen against fighters, while the Bulwarks were concentrated around critical areas to defeat missiles. In time, however, its numbers would increase to combat the effectiveness of Covenant plasma missiles and strikecraft.

There were also two large launchers that were used to carry the heavy M81273 Stormbreaker fusion rockets. Little more than a high-performance fusion drive and its fuel cell, this was by far the longest-ranged and heaviest-hitting weapon that the Endeavour boasted. Within a short time, it could reach velocities that crossed hundreds of kilometers within a second, and their kinetic warheads inflicted mortal wounds on anything that it happened to hit. These were rarely used, however, as their unguided design meant that they could rarely hit most mobile targets except when they were carefully aimed and fired. They were also hugely ineffective against energy shields, though they were still carried to make up for the destroyer's lackluster MACs.

The 2553 refit had completely changed this. The twin MACs were now the Mark IX, light coil MACs, which were a total upgrade over the old Series-2. Most significantly, they recycled about 35% of the energy used in the firing process back into the capacitors, enough to allow it to fire an extra shot per charge. The venerable M66 Sentries were finally removed and replaced with triple-turrets equipped with the new M74 Sentry system. M58 Archer missile pods were still present, but their numbers had been increased to 24 pods. The refitted Endeavour was bristling with dedicated anti-fighter weaponry: 48 cells of M340A4 Streak missiles, 8 M910 Rampart 105mm guns, and no less than 120 M710 Bulwark 25mm guns were present to obliterate any space fighter attack. Two silos for the Ragnarok-class nuclear missiles, each with a 40-megaton nuclear fusion warhead, replaced the fusion rocket turrets that were fitted. These were considered weapons of last resort against opponents that significantly outclassed the destroyer, and their EMP qualities made them much more viable against Covenant warships. All these changes leaned the destroyer much closer to the air defense role than it ever did before.

Refits
"What you are looking at is the last original part of the ship. Every other wall, beam, and electronic has been replaced or rebuilt three-times over."

- Petty Officer, 3rd class Michael 'Stuck' Bennet

As a former CMA warship, the Endeavour was typically passed over by UNSC officials when it came to upgrades, with any improvements made coming from reclaimed parts that belonged to scrapped vessels. This was partly fueled by their disapproval in combining the roles of destroyer and transport, as it was not able to fulfill either role as well as many other vessels already in service. However, as it continued to survive engagements with the Covenant, refits were eventually authorized that steadily built upon its capabilities, with these often being implemented between large-scale repairs.

By 2528, the Endeavour had received a small suite of improvements, as five years of constant action would obviously cause severe wear on its systems. While most of its systems replacements were minor and hardly affected its performance, with its powerplant, electronics, and even its superstructure being modified to some capacity, two significant changes were made. The first was the installation of a TN-91sy/n Triad targeting suite taken from the, an older computer network that allowed for better coordination of nearby Archer missile swarms and plotted more accurate fire from its naval autocannon battery. The second was a controversial array of radiators at exposed parts of the ship, which while it enabled the Endeavour to overclock its reactor and engines well above their designed limits, left holes in its armor that was never fully addressed over its lifespan. The only other noteworthy change was the increased complement of point defense guns, in a desperate attempt to better protect it from plasma torpedoes and the unexpectedly-resilient Covenant strikecraft. Because the shipwrights had to rely on whatever weapons were available, it was not composed of one homogeneous type of turret, including others such as the M810 Helix and various models of Rampart guns. A number of other weapon systems were temporarily fitted - the Endeavour at one point boasted seven silos for M280 Ares missiles sloppily fitted onto its keel, which were so rarely rearmed that these were removed during the 2528 refit.

In 2528, after suffering extensive damage following the Battle of New Tharsis, the Endeavour limped to the regional shipyards of Passage, where it underwent its first major refit. This was an expansive affair, involving the modification of vital internal equipment, the movement or mutation of its armor and hull, and the removal of dozens of makeshift repairs made over the last seven years. Most prominently, its forward bow was angled downwards and expanded in size, originally to allow for the installation of a third MAC. However, high demand for such weapons saw the space being given over to supercapacitors, munition storage, and more accommodations for Marines and pilots. Personnel assigned here were nicknamed the 'poundees,' as the firing of the MACs would rock or pound the entire deck when they fired. The last change were the 8 quad M112 Viper 375mm/26 cal naval coilguns which were fitted beneath the primary MAC, offering a substantial increase in firepower.

No new major refits would occur between 2528and 2552, as older material from the CMA was being quickly expended during the remaining battles in the Outer Colonies, while new material was diverted to the construction of new starships. Once again, any changes were made during repairs, which typically had been higher-quality than those that had been made before. An additional six M58 Archer missiles were added, while 48 M340A4 Streak missile silos provided enhanced counter-fighter defense. Various improvements over time strengthened the hull, and the ineffective VEIL jamming suite was finally removed and replaced with a Mark XII GREY MIST wide-spectrum jamming computer. The engines had been exchanged on no less than four occasions. The newest ones, four ILT-DTF-9 fusion drives, were secondary units that were taken from a scuttled heavy cruiser, and resulted in a thrust increase of 45% over the original models fitted onto the Endeavour.

After the Human-Covenant War, the UNSC Navy authorized a series of enhancements to first fulfil its need as a test vessel for Project SPHINX, and later expanded into a full-blown rebuild in 2554. Almost every single component of the destroyer was gutted and replaced, which drastically increased its performance in all areas and made it competitive with far more modern warships. While too expansive to list each change individually, there were some highlights. Under SPHINX, the Endeavour received a Kaalt Nu-pattern borer, a small Covenant-built faster-than-light engine. This changed the destroyer's role into a rapid response and messenger ship, as it could effortlessly outpace even the newest CODEN drives that were just entering service. It was also under the program that a Rutherford DIS-3081b compound fusion reactor was installed, a completely-new powerplant that was a scaled-down version of the model fitted onto the UNSC Pillar of Autumn. This allowed the Endeavour to overclock its reactor by over 300% for short periods, as it did not have the advanced cooling system to nullify overheating.

Construction and commissioning
The destroyer that would be christened the Endeavour first began construction in orbit of Chi Rho by the local construction company Diadel Systems, who had been subcontracted by SinoViet Heavy Machinery to construct a number of modified destroyers for the Colonial Military Administration. It was first laid down on August 22, 2518 as the third of the thirteen destroyers built of its batch, which ended with the completion of the CMA Heart of Midlothian in late-2522. As its previous two sisters allowed Diadel to identify and fix many issues with manufacturing and shipping of parts, the destroyer suffered no construction delays and even finished fabrication two months ahead of schedule. It was launched on January 27, 2519 to complete the fitting-out process.

As construction came to a close, the destroyer was subjected to a rigorous set of space trials, intended to test the operation of its systems and general integrity. This consisted of a series of hard burn accelerations, sensory tests, and low orbital passes. Although these were generally quiet affairs, the third of these trials was noteworthy. The ship was caught in an intense comet bombardment that saw shards of rock impact the destroyer's hull, which forced it to conduct extreme evasive maneuvers to escape damage. One of these turns saw it unintentionally collide with the MSV Ivory Tower, a large maintenance tug that was sent to rescue it. The ordeal would delay its commissioning, but the minimal, almost superficial extent of the damage showed that the destroyer was much more ruggedly built than was first believed by its builders. It was commissioned as the CMA Endeavour on March 19, 2519, and was delivered to the CMA a mere three days later. It then departed the Ectanus 45 system, manned by a rookie crew under the command of Lieutenant Commander Asia Vespa.

Insurrection
"Kids on Fjord fight better than you."

- Chief Warrant Officer 4 Kores Van Schalkwyk, to the Endeavour 's insurgent crew.

The CMA Endeavour first served in the 173rd frigate flotilla, a light naval unit that operated in the Lurone, Rawadi, and Rowena Sectors of the Doradus-Scorpio Quadrant. In this guise, it was primarily used to conduct light troop movement through the region, where it ferried elements of the 276th Colonial Army Battalion towards unrest hotspots on Pumice and Cadence. It lent naval support where possible, but thanks to the small-scale of the fighting, its support was generally limited to logistics and coordination. Other tasks included convoy protection duties to the habitats of Campbell's Landing, in an effort to deter losses to piracy in a navigational hazard zone around it. Most of these tasks were quiet affairs, as nobody realistically wanted to pit themselves against a Colonial Navy destroyer. Instead, it generally acted as a glorified rescue ship, rescuing numerous civilians and businessmen whose ships were stranded or forced to be abandoned.

In October 2521, the Endeavour was reassigned into the 114th Rapid Strike Squadron, the largest and most active CMA naval unit in the Asgard Stratum, as a replacement for the destroyer. This reassignment was also to partially cover the refit of the carrier, with the Endeavour taking over some of its servicing duties for the four months it was in dock. Within the 114th, the destroyer was supposed to assist any starships in distress, where it would quickly move to dock and embark any crew and cargo that can be saved from disabled craft and fight off any attackers. The agricultural content of these freighters meant that it was vital that shipping losses were kept to a minimum, and the Endeavour was involved with the rescue of 19 civilian freighters and four warships during 2522 alone. However, the 114th's power meant that its escorts were responsible for tracking and hunting down any significant insurgent elements in the region, as well as reinforce any CMA garrisons that were struggling to put down unrest. For this need, the destroyer participated in putting down the insurrections of Madrigal, Green Hills, and especially Redstow VI, where it supported ground incursions with orbital strikes at the cost of escalating civilian casualties. However, the sudden increase in readiness and activity, as well as the now-notoriously low standard of training given to CMA recruits, meant that the Endeavour 's largely-inexperienced crew were seen as both incompetent and lazy. This was almost dangerously so, as it almost fired upon allied vessels on several occasions; this reputation would only be erased following the start of the Human-Covenant War.

In February 2523, Vespa stepped down as the Endeavour 's commanding officer, and was replaced by Lieutenant Commander Chi Kariuki. An ambitious officer, it was under her leadership that the destroyer became host to sympathizers of the Biko Independence Army (BIA). At first, the only assistance the crew offered were leaked communiques sent through the squadron, and Kariuki's desire to impress saw these quickly shut down. It was only through negotiations and bribery of her communications officer, Lieutenant Patryk Adamczyk, that operations escalated. Throughout the rest of the year, the Endeavour actively assisted the BIA's efforts by providing dead drops of infantry equipment and firearms, starship munitions such as Archer missiles, and even entire vehicles and dropships when possible, all of which was smuggled back to Biko. This assistance was also afforded to other Insurrectionist groups, where it misdirected CMA and UNSC forces by providing false sensor reports. It even faked the deaths of Godfrey's Commandos, one of the Secessionist Union's most elite forces, during Operation: THUNDERBIRD by quietly folding them into the crew as their own department.

All of this eventually culminated in a scandal. In early 2524, the BIA used many of the weapons they received from the Endeavour in their fifth uprising, and were swiftly put down by UNSC forces. Analysts from ONI Section One were brought in to scour through the wreckage, as well as interrogate one of the captured insurrectionists for information. While they were able to determine that the equipment was stolen from the CMA, the exact origin was ultimately unknown as their prisoner did not survive. As a result, ONI covertly seeded teams of investigators among CMA garrisons to hunt it down. One of these teams, composed of Chief Warrant Officer 4 Kores Van Schalkwyk and his Navy handler Lieutenant Theodora Papadopoulos, were officially serving as trainers for the Endeavour 's when they were captured by the ship's crew. It was only when they escaped and intercepted one of the weapons drops left behind that they obtained solid proof that connected the destroyer to not only the BIA, but also exposed the rampant insurgent activities in the region. The Endeavour was immediately suspended from operations, and its entire crew were detained and interrogated by Section 1. Many would be pressured into resigning from the CMA, LCDR Kariuki and several of the senior officers were court-martialed for negligence, and the few that were released were reassigned. The ship itself was thoroughly swept of any listening devices, and its databanks were recovered for further analysis. When it finally returned to service in January 2525, it would be under an entirely-new crew.

Covenant discovery
The CMA Endeavour 's first actions in what would become known as the Human-Covenant War occurred in May 2525, when it was sent to investigate the loss of the CMA cruiser Urban Encroachment in the Upada Sector. A four-week search was extended into six when a second starship, the CMA Empyrean Jewel, disappeared in the same region. No traces of either starship were discovered, something which led to the regional command theorizing that they had been hijacked. As a result, it was ordered to rendezvous with the frigate CMA Raltstown to escort merchants through the Asgard Stratum in hopes of deterring raids.

However, when the civilian craft of the CAA Astrography Charting Mission deployed to the Aurna Sector failed to check in, the entirety of the 114th Rapid Strike Squadron was sent in to locate them on July 13. After 16 days of searching, the Endeavour recovered Item-291X - an alien fragment that appeared to be have once been part of a ship's hull. Two days later, the destroyer met up with another CMA corvette to verify the discovery of the wreckage belonging to one of the Mission's command ships, the CAA Babyloniaca. Preliminary analysis of the damage was undertaken, where it was discovered that it had been subject to an intense bombardment of high-temperature plasma, which breached all decks and fused the propulsion systems. More importantly, the Endeavour 's weapon officers and chief engineer both noted that the damage could not have been caused by any known weapons they were aware of. The wreckage was taken back to their home base at Biko so it could be analyzed further. Both Item-291X and the Babyloniaca convinced the regional CMA command to keep the 114th Squadron in the area, as well as reassign the smaller 38th Scout Flotilla to assist them.

On October 26, the Endeavour participated in the Skirmish of CRT-99-LP. It arrived into the system to exchange supplies with the corvette CMA Johnston, only to find itself suddenly attacked by what was later identified as a. In an action lasting a minute and a half, the destroyer took several blows from the much smaller alien ship, breaching D-deck on the bow and killing four people. Meanwhile, the only hits from the human side were from a salvo of one of the Endeavour 's naval autocannons, with its missiles rendered completely by its jammers. While it did force the raider to flee, no damage was registered on the enemy ship. The destroyer immediately began a rescue of the Johnston 's 13-strong crew, although only a single comatose survivor was recovered.

The Endeavour towed the Johnston back to the nearest CMA outpost at Green Hills, where they filed an urgent report to CMA Command concerning the engagement with an unidentified alien vessel. They were instead surprised to find that they received orders by the UNSC High Command, signed by the UEG Minister of Defense, to head with best possible speed to the nearest UNSC naval base at Falaknuma in the 18 Scorpii system. Upon arrival, the two ships, along with the recently-arrived CMA Hercules, were temporarily grounded as ONI Section One teams combed over them for any intelligence on their potential adversary. These included thorough interrogations of the crews, reviewing computer readings taken during the battle, and inspecting the damage incurred to discover new insights. Repairs were halted while this process was ongoing.

In light of ONI's conclusions and the discovery of what happened to Harvest, the UNSC declared a state of emergency on November 1, forcibly placing all CMA personnel and property under its direct jurisdiction. As a result, the destroyer was rechristened the UNSC Endeavour, and major software upgrades were implement to allow it to interface with the UNSC battlenet and communications systems.

Cole Campaigns
The UNSC Endeavour, along with the other few warships that had already fought against the Covenant, were reassigned into the massing Battle Group X-Ray. The newly-returned Vice Admiral Preston Cole specifically requested these ships in hopes that their crews could better inform his upcoming strategies. In any case, the destroyer participated in the Second Battle of Harvest, where Cole's fleet attempted to destroy the single that orbited the planet. It was first tasked with screening the larger cruisers from incoming plasma fire, but then moved forward when it became clear that the fleet's countermeasures were unless against the Covenant warship. It and the other destroyers tried to take advantage of the battleship's focus on X-Ray's capital ships by swarming it, but the near-misses and glancing hits kept the Endeavour away while its shields easily withstood their fire. The entire fleet was almost about to retreat when Cole ordered the entire fleet to target a specific part of their opponent's bow, and coordinated their salvoes so they would hit at roughly the same time. This overwhelmed its shields and quickly led to its destruction, giving the UNSC its first major victory in the Human-Covenant War.

The UNSC Endeavour participated in the Battles of Green Hills and Alpha Auriga with Battle Group X-Ray, but the small size of its MACs eventually saw it transferred into a support role. Instead, between October 2526 and February 2527, it served in a wolfpack with two other Halberd-class destroyers, where they departed from Harvest to raid Covenant logistics lines. They claimed to destroy thirteen ships over that four-month period, but further success was halted when the flotilla was lured into a trap. The Endeavour was the only survivor of the bloodbath, as it was holding back from the main force when the attack occurred.

The Harvest Campaign
Once it finished repairs in June 2527, the destroyer was assigned to Battle Group D, who were charged supporting the invasion of Harvest. Although the system was not hotly contested by the Covenant, there were still a few holdouts that were stubbornly entrenched on its surface. Before it even arrived, the UNSC Endeavour had already saw action when it intercepted a small, lightly escorted supply ship, though after an extended chase the vessel ultimately escaped.

Once it established orbit, the destroyer supported ground offensives in the Western Edda region by providing a mixture of fire support, imaging for reconnaissance and navigation, and acting as a forward operating base away from the theatre's assault ship, the. It directly participated in Operation: RIVER DANCE, where it deployed teams of ODSTs to find and destroy known Covenant communications and sensor stations, allowing the troops below to make a surprise march on the city of Thrudheim. It directly attacked a number of smaller regions, particularly those that were protected with anti-air defenses that could keep away other frigates. The few Covenant excavations sites that were captured intact were carefully examined by ONI officers on the destroyer, although they were unable to find out what they were looking for.

In November 2527, a massive Covenant counter-attack of Harvest changed the campaign for the worst. The Covenant's Fleet of Valorous Consecration appeared in force and clashed with the UNSC's fleet for control of the system. Battle Group D began to offload as much personnel and supplies as they could before withdrawing, and the Endeavour worked with the other frigates to destroy as many of transports and fighters as they could. The Covenant retaliated in kind; a wing of 213 strikecraft attacked the destroyer and inflicted damage, but support arrived quickly enough for the Endeavour to remain operational. Large sections of Western Edda had to be abandoned as it could no longer be maintained without orbital support. Finally, on December 3, Battle Group D was forced to withdraw as three arrived to decisively remove any UNSC presence in orbit. As one of the last vessels to leave, the Endeavour was pursued by one of these destroyers, although late support from the cruiser saved it from destruction. It was towed back to New Tharsis for repairs, unknowingly being tracked by the Covenant.

Battle of New Tharsis
The Endeavour was reunited with the 114th Rapid Strike Squadron for the first time in three years, when the sole surviving vessel, the, limped to New Tharsis for emergency repairs and reinforcements. While the 114th was awaiting for replacement ships, New Tharsis came under siege by the survivors of the Fleet of Valorous Consecration, which were now tasked with causing as much damage in the immediate region as possible. The shipyards of New Tharsis, and the factories below, were the beating heart of the UNSC's war effort in the Asgard Stratum, making it a prime target for the Covenant armada.

The Endeavour participated in the first battle between the UNSC defense fleet, divided between Commodore Admiral Ashok Raymond's 114th Squadron and Rear Admiral India Martel's Battle Group Roma, and the Fleet of Valorous Consecration. This costly engagement managed to beat back the Covenant, but was unsuccessful in stopping one of their from reaching the ground. In the aftermath of the battle, Ashok transferred to the Endeavour to conduct a search-and-rescue operation, one that was cut short when the Covenant attempted a second attack. Holding back to coordinate the fleet, this left it in the prime position to pursue when another enemy corvette made a break for the surface. It and the UNSC frigate Winter's End both followed the Covenant vessel through New Tharsis' atmosphere, fighting in a running gunfight. Though the Winter's End had to disengage due to damage, the Endeavour maintained pursuit, and they eventually reached the Covenant's primary base of operations on the surface. Under simultaneous fire from anti-aircraft guns and the two Covenant corvettes that bypassed the UNSC fleet, the destroyer was forced to withdraw, though it deployed a nuclear warhead that annihilated the base and damaged the two enemy ships.

With their primary base destroyed, and the UNSC fleet in orbit crippled from the last engagement, the Covenant invaded with the rest of their ships to reinforce their holdings. With the orbiting capital ships unable to assist, Raymond transferred his flag to the Endeavour permanently, and used it to coordinate naval and air support. Although it fought in a number of minor battles, such as when it held off Covenant fighters to allow the Drawn maritime fleet to escape, the rapid Covenant advance meant that it was held back until the Battle of Obsidian, the planet's capital city. Supported by the remaining fighters, heavy frigates, the Endeavour clashed for superiority with the Covenant's three remaining escort ships for air superiority. The superiority of the alien warships forced the Endeavour and its flotilla to fall back closer to the city limits, under rapid pursuit. This was, however, a feint: Raymond wanted the Covenant ships to be drawn deep into range of the well-camouflaged planetary defense guns, which suddenly opened fire. Under fire from all directions, and the fleet rallying under a new attack plan, two of the Covenant ships were quickly destroyed while the third escaped. Their lent fire support quickly routed the Covenant ground troops from the city.

The destruction of the three enemy ships had profound implications for the rest of the campaign; whenever because of damage or being destroyed, the Covenant's only intact ships were all heavier units. Without the speed needed to catch the UNSC ships, the Endeavour and the other frigates were ordered to support ground operations and raids, usually to draw the Covenant ships and fighters away for various ground operations. It participated in the Battle of the Emerald Sea, where it and maritime ships succeeded in bringing down the Covenant's sole CCS-class battlecruiser.

The Years of Hell
"These are hard years. We've lost so many good people, many of them close friends, and it is taking their toll on me and the crew. Everyone knows that they will probably die in the next few weeks, and I cannot feel anything when I add an acquaintance's name to the casualty lists. I wish I could say that their deaths meant something, but I know that's a lie. I don't know how to maintain morale in these kinds of conditions."

- Excerpt from the personal log of CDR Aditya Misra, describing her brief captaincy on the Endeavour.

Following its major refit, the Endeavour and the rest of its squadron were briefly deployed in convoy search and escort duties. They were only able to search a handful of freighters before a new front had opened up in the Armstrong Expanse, requiring all available ships to be consolidated into a new fleet unit - the 16th Defense Fleet - to counter the Covenant advance. This period was one of intensive combat as the UNSC struggled to hold onto their territory. In the rest of 2528 alone, the Endeavour participated in over three dozen engagements, with most of these being aborted attempts to board enemy warships in hopes of collecting intelligence. However, it did assist in evacuating over 370,000 people from Campbell's Landing, where it directly protected the civilian convoy while three capital ships held off six CPV-class destroyers.

The Endeavour was part of the squadron that escorted the remnants of Battle Group Tsushima back to Nuevo Cuzco, who had been swiftly beaten at Midvale. This led to widespread food shortages throughout the sector that greatly impacted the morale of its crew. It later participated in the battles of Hat Yai, Cadence, and the Indra Asteroid Belt, where although it was supposed to be part of the transport groups, the UNSC's inability to maintain orbital superiority brought it directly into combat time and again. The 16th Fleet became so desperate for ships that the Endeavour never stayed in dock for any longer than a month, with most of the repair work rushed to return it to the frontlines. This was so poor that the hull would crack when entering a planet's atmosphere. So much work had to be done that its damage-controls team were nicknamed the 'sleepless ones' by the rest of the crew.

Things only intensified from 2532-onwards. During Operation: MOONLIGHT, the Endeavour supported the 16th Fleet's efforts in reclaiming Hat Yai by providing troops and fuel for prowlers. It later fought in the Second Battle over the colony, where it escorted the assault ships through the hole in the Covenant's defenses. Although some initial success was achieved, with ODSTs sabotaging most of the war industry present, the operation was quickly shut down as losses reached unacceptable levels. This led to a ban on offensive missions, with only prowlers granted an exception. As a result, all naval ships in the Armstrong Expanse were ordered to Kingston.

The subsequent Battle of Kingston was a massive campaign that attempted to hold back the Covenant's expansion. As most of the prowlers were deployed to raid enemy logistics, minelaying was instead transferred to the frigates and destroyers of the 114th Squadron. The Endeavour seeded minefields around identified jump points and travel zones using 500-kiloton non-nuclear explosives, and while a number of kills were scored, the destroyer endured heavy resistance that sometimes forced it to abort the operations. As the battle progressed, it shifted to escort missions to deter attack ships, taking heavy damage in the process. It was during the latter that it suffered a near-catastrophic hit from a plasma beam that crippled its MAC and gutted its keel, requiring it to pull out of the battle to receive repairs at Hollow Point.

The Endeavour was still undergoing repairs when the Battle of Kingston drew to a close, and what remained of the 16th Fleet retreated to Hollow Point. It briefly fought against the Covenant when they arrived only days later, but with its MAC still not online and the UNSC Navy taking heavy losses, crew morale was pushed to breaking point. Eventually, its bridge took a direct hit and killed all the senior officers, including Captain Aditya Misra. With her to enforce order, the Endeavour swiftly found itself at the center of a mutiny which installed Lieutenant Govind Chaudhary in command. He ordered the ship to disengage from the battle, charting a course using the Cole Protocol to evade the Covenant and eventually reach Human Space again.

SPHINX involvement and recommissioning
In the aftermath of the Battle of Earth, UNSC engineers were sent to the Endeavour 's crash site to evaluate what components could be salvaged for new construction programs. Their analysis discovered that, aside from the keel, the overall structure of the destroyer was in surprisingly good condition, and they recommended that it be raised and brought back into service. After nearly a month of repairs, the Endeavour 's engines were fired once more and, with assistance from nearby tugs, it was led into an orbital drydock. However, work paused once it was discovered just how old many of its components were. While a carrier destroyer was considered useful, the cost of its repairs and upgrades made it unattractive to the UNSC Navy's budgetary officers, who debated about whenever to retain it or sell it off for scrapping.

The fate of the old warship was put to rest when Project SPHINX acquired the hull. After being moved to a backup facility at Titan, the Endeavour was given patchwork repairs and converted into a technology test ship, which it would serve as for the next two years. It received Covenant antigravity plating and a small borer taken from a captured, and put through rigorous experiments that tested the borer's capabilities and reception to UNSC control systems. This included several dangerous atmosphere-to-atmosphere transitions that almost tore it apart, although improvements from SPHINX's engineers would make later trips much more comfortable and safer. It also experimented with a number of other components such as plasma torpedo silos, where it was found that the destroyer's reactor did not have the output to allow it to function. This led to them replacing it altogether with an upgraded model. The torpedo tests were a failure, as on two occasions, they failed to respond to the Endeavour 's guidance signals, while on the third the munition overcorrected and almost hit it.

With their desired experiments completed, SPHINX formally returned the Endeavour back to the Navy on September 4, 2554. Although they removed much of the equipment they installed on it, the borer was left intact in order to allow it to act as a keystone ship in UNSC fleets. The Reyes-McLees Corporation were contracted to fully modernize it at their Martian shipyards. The Endeavour was formally recommissioned and handed back to the UNSC Navy on November 18, and left the shipyard under the command of Commander Tom Žagar.

Crew
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Captains

 * CPTN Taylor Gai (January 2519 - March 2519): Commanded her during her space trials.
 * LCDR Asia Vespa (March 2519 - February 2523): Transferred to command the frigate CMA Swipe Left.
 * LCDR Chi Kariuki (February 2523 - October 2524): Court-martialed and dishonorably discharged by ONI.
 * LCDR Costică Ionescu (January 2525 - January 2526): Killed during the Second Battle of Harvest.
 * CDR Wieland Hauer (March 2526 - February 2527): Killed during the Skirmish of CP-99 system.
 * LCDR Anargul Eldar (May 2527 - 2531): Field-promoted to the rank of Commander and transferred to Naval Command of New Tharsis.
 * CDR Aditya Misra (September 2533 - July 2535): Killed during the Battle of Hollow Point.
 * LT Govind Chaudhary (July 2535 - August 2535): Acting Captain, imprisoned after his crew mutinied against him.
 * LT Lochana Rao (August 2535 - November 2535): Acting Captain, relieved after returning the Endeavour to port.
 * LCDR Róża Stawski (January 2535 - April 2537): Killed in action.
 * CDR Shannon Hambleton (2541 - 2545): Promoted to the rank of Captain and transferred to the.
 * CDR Tom Žagar (2553 - present): Current serving CO.

Bridge officers

 * LT Patryk Adamczyk (September 2522 - October 2524): Communications officer, discharged by ONI.
 * LCDR Harry Hamilton (August 2523 - November 2529): Weapons officer, killed in action.

Miscellaneous
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 * CWO4 Kores Van Schalkwyk (March 2524 - October 2524): UNSC technical liaison and trainer, transferred following the completion of his mission.
 * LT Theodora Papadopoulos (March 2524 - October 2524): UNSC naval liason, transferred following the completion of her mission.

Mascots
The Endeavour 's first mascot was a female diamond python named Scarf. Confiscated after a civilian freighter was searched in 2528, her discovery was never reported and she was swiftly adopted by the engine crew. Known for her placid behavior, she was remarkably easy to handle and liked to intertwine herself in the staircase railings. Her complete lack of any threatening behavior was why her handlers occasionally slipped her into the quarters of various crewmembers for a joke. In her later years, it became a tradition for her to be slipped on the shoulders of new engineering personnel. She was killed during the Battle of Hollow Point in 2535.