Antecedent

Antecedent civilisations (or Antecedents) are a broad-sweeping classification, used to describe civilizations which have since gone extinct. The most well-known of such civilisations include the Forerunners, who were responsible for destroying all life in the galaxy with the firing of the Halo Array. As such, the vast majority of known antecedents are those civilisations which fell alongside the Forerunners' empire - though the term has also come to include a number of extinct civilisations whose remains have been found littering the Milky Way unrelated to the Forerunners. The majority of these examples come from the explorative efforts of the Covenant, though a handful are known to humanity.

More recent revelations about the Forerunners now postulates that the Forerunners were far from the first major spacefaring empire to dominate the stars, but rather merely the tip of a large iceberg stretching back billions of years. Evidence of these even older and more ancient interstellar communities is even harder to come by than the already-rare Forerunner relics, though this theory has been proposed as the explanation for a number of aberrations in what are believed to be Forerunner sites and artefact collections.

Antecents have some overlap with the grouping of Ulteriors, though are primarily distinguished by their civilisations being wiped out and known only via their relics.

Pre-array
With evidence increasingly pointing to a galactic starfaring history stretching back hundreds of millions - if not billions - of years, the vast majority of antecedent civilisations existed prior to the firing of the array. The vast majority were likely swept up in the marches of time, with only a handful of a percent expected to ever be recorded in the modern day. The Forerunners were the last in this line of galactic civilisation and as such, the majority of antecedents from this era are those which had contact with - and were recorded by- the Forerunners.
 * Ancient Sangheili civilisation - Prior to the firing of the Halos, the Sangheili were known to have served the Forerunners as subjects within the ecumene. In this role, the Sangheili are believed to have served as levies within the military, paying tithe in return for the Forerunners' protection.
 * Ancient San'Shyuum civilisation - Similarly to the Sangheili, the San'Shyuum are known to have had a rich starfaring history prior the Great Purification. Unlike the Sangheili, evidence recovered from Forerunner databanks suggests the San'Shyuum once held a small interstellar empire in the Milky Way and were seen as rivals to the Forerunners' power, ultimately resulting in a series of wars with the Forerunners eventually seeing the San'Shyuum confined to just their home system under Forerunner rule. During the conflict against the Flood, the San'Shyuum rose up in rebellion against their distracted overlords.
 * Dyson engineers - A culture known of only via inference, this unknown civilization is assumed to have constructed the slipspace-based Dyson shell later appropriated by the Forerunners as the core component of the shield world Onyx millions if not billions of years ago. As the engineers are known only by this single relic, it is possible that they are synonymous with another civilization, such as the Precursors.
 * Ehrlon - the Covenant name for an interstellar culture who were active around 120,000 years BCE in several dozen systems in the region of the Orion Arm known to the Covenant as the Niphos Barrens. They are assumed to have fallen victim to an unknown cataclysm, leaving only ruins behind.
 * Endless - Active in the galaxy around 250,000-160,000 years ago, this civilisation is referenced only a handful of times in Forerunner texts. While information on them is scarce, current theories suggest this civilisation may have once been the dominant galactic power prior to the era of the Forerunners.
 * Forerunners - The most well-known antecedent example, the Forerunners dominated the galaxy in the millennia leading up to the firing of the Halo Array, and left their artefacts scattered across the galaxy to be found by their successors.
 * Harhatri maze builders - An enigmatic civilization that constructed the vast network of stone-mazes and catacombs of Harhatri millions of years ago. The ruins, which are uncommonly gargantuan in scale, reportedly lack visible signs of technology, are variously said to be haunted, or possess time-dilating properties to those foolish enough to explore them.
 * Niihkta civilization - An ancient culture once inhabited the methane-rich moon of Niihkta, located in the Ryhon Tangle. Though no intact technology survives in the moon's harsh conditions, which appear to have exacerbated in the last 100,000 years, the surface is still littered with the remains of well-developed cities and various lesser communities. The civilization appears to have mastered at least atomic energy, though if any space presence existed, it is now long gone. The most curious feature of the ruins are carvings depicting beings remarkably similar to the Unggoy. The ruins are generally agreed to be nearly 200,000 years old, far predating any known Unggoy expansion. Though the nature and even exact dating of the Niihkta culture are a matter of dispute among Covenant scholars, with some going so far as to call the carvings a later forgery, it has been theorized that they may be evidence of a hitherto-unknown Unggoy splinter culture, perhaps one displaced from Balaho by a precursor civilization.
 * Orb-builders: A culture known to the Covenant by several apparently inert sphere-shaped megastructures they left behind millions of years ago, nothing else is known about them. Orb-builder spheres are recognizable by their geometric precision, highly exotic construction material and baroque surface patterning down to the micrometer scale. Any visible technology the orbs may have contained has either been looted over the eons or was never there.
 * Precursors - A semi-mythological race mentioned by some Forerunner records, the Precursors were hypothesized by the Forerunners to have lifted earlier Forerunner ancestors from Earth and displaced them across the galaxy hundreds of thousands of years ago. Assuming they existed as a singular civilization, the Precursors may be the explanation for most cases of out-of-place biota.
 * Seeders - A hypothesized race based on the findings of a remote research team in the Skaldi 342 system. ONI listening telescopes detected signs of a particularly violent meteor collision in 2547, leading to recon probes and later remote contact teams being sent to investigate the curiosity. What the teams found was a non-native fragment of the impacting agent which had survived the relativistic impact, which was later determined to be at least twenty-five million years old. The fragment was theorised to be part of a von Neumann probe launched at slower-than-light speeds across the gulf of interstellar space, giving rise to the species' nickname.
 * Sculptors - Known only by the sculpted asteroids they left behind in several systems in the trailing side of the Orion Arm, the Sculptors are hypothesized by Covenant scholars to have been active over a hundred million years ago. Sculptor relics contain no useful technology and their purpose is theorized to be artistic or religious in nature.

Post-array
During the reseeding process following the end of the Diluvial War, the Librarian seeded new populations of dozens of sapient species on thousands of worlds across the galaxy. This was done in an effort to ensure that a newly-reintroduced population would not be rendered fully extinct through any of the dozens of Great Filters that could potentially halt the rise of intelligent civilisation - and is a part of the reason for the out-of-place biota phenomenon. As predicted, many of the species that rose up during the following millennia were wiped out through any number of causes both self-inflicted and natural.

The explorations of the Covenant have recorded a small handful of worlds which appeared to suffer such fates, with several of the Covenant's own client species including the Unggoy and the Jiralhanae having come close to joining the list due to overindustrialisation and war.

Humanity has only discovered one such world, though the inevitable expansions of humanity - particularly into the spin-ward regions of the Orion Arm - is expected to slowly reveal more and more lost civilisations to xenoarchaeologists.
 * Jehioi - A civilisation known by the Covenant who established planetary and orbital colonies across space in the Orion and possibly Perseus arms, before falling less than two millennia later. The Jehioi are estimated to have been predominantly active around 40,000BCE. The Ior have been theorized to be a primitive offshoot population of Jehioi or a related species.
 * Line Installation 5-16 wreckage - A line installation located within Covenant space. When the installation was discovered by the Covenant in 1534CE, the installation lay dormant, allowing the Covenant explorers to land freely. The installation's surface is covered in the remains of hundreds of starships of alien origin, including those of Forerunner and non-Forerunner origin. Unfortunately, any chance to glean much useful data about the site was lost due to site having been scavenged by the Optem some time prior, with all immediately useful technology stripped - leaving only the husks of empty starships in their wake. Nonetheless, analysis of the basic construction of these ships suggests the wreckage comprises around five or six civilisations whose spacecraft had been captured by the line installation prior to the facility's deactivation - presumably by the Optem. The craft range in technology level and ergonomic design, though some evidence suggests a handful of the craft may originate with the same species - though with massive differences in their technological development level, suggesting the craft were captured by the installation over the course of thousands of years.
 * Optem - A species nicknamed the "Looters", this civilisation appears to have been active around 20,000BCE and is known to have raided Forerunner sites for their treasures - hence their nickname. Whether this civilisation corresponds to any of the others known in the galaxy, or whether they remain active, is unclear.
 * Rhiln - An Ulterior civilisation discovered by the Covenant during the 2nd Century CE, the Rhiln were the subject of a war of extermination by the Covenant during the 250s and 260s. The Rhiln were initially on the winning front due to their advanced usage of artificial intelligence and dyson swarm construction, but Covenant forces were ultimately able to penetrate into the Rhiln home system and shatter their dyson swarm. A handful of Rhiln that had resisted joining the Rhiln compound mind were spared and ultimately found place in the Covenant fringe as a minor species.
 * Silent Hope - Unlike the others on this list, this antecedent takes the form of a Covenant civilisation. At the time of its settlement in 480BCE, the colony of Silent Hope was viewed as a new cultural nexus in what was then the outer frontier of Covenant settlement. Reaching a population of millions at its height, the world was later entrusted to fend for itself during the waning period of the First Illumination, during which time Covenant civilisation looked inward. By the year 30CE, no confirmed contact had been made with the world for eight decades, and scout missions were sent out to ascertain the planet's status, but only found the world totally abandoned, as though the entire populace had evacuated without packing their belongings or - in some cases - even finishing their meals. No bodies were ever found, and Silent Hope's parent star system became the subject of legend. Even despite the system's ample resources, all that remained in the system was a single mostly-automated helium refinery orbiting the gas giant for refuelling of errant freighters passing through the system.
 * Netherop civilisation - a seemingly-(relatively) primitive civilisation whose ruins were discovered on the eponymous planet. The planet's discovery was relatively late in the 25th century, and included in the Military Survey of Uninhabited Planets as a greenhouse world with little of value to justify infrastructure investment in the Ephyra system. ONI-classified addendums to the report showcased several photographs taken via remote survey probes of the planet's surface, highlighting what appeared to be evidence of city-like structures on the surface, though the near-Venusian surface conditions made practical exploration and research difficult. Netherop was a site of minor interest to the Covenant early in the war against humanity, though the rapid retreat of UNSC forces meant that no major ground exploration was ever conducted. Photographic and video recordings taken by Spartans deployed onto Netherop early in the war have led xenoarchaeologists to estimate that the native civilization on Netherop had at least attained an early industrial level of technology before being wiped out around 30-45,000 years ago.