Sanghelios

Sanghelios is the homeworld of the Sangheili, and was one of the most influential worlds in the Holy Ecumene.

Primeval Period
During the 3rd millennium BCE, Sanghelios enjoyed an age of unprecedented technological advancement and societal progress. Gleaming megacities supported a planetary vast population, with a growing number of Sangheili living in orbital habitats and even a handful of off-world colonies. This period, known as the Age of Decadence by latter-day Sangheili, was brought to a screaming halt around 2,300 BCE. Although records of what exactly happened are now lost to history, some sort of experiment caused the Shard of Heaven to malfunction. This triggered the Skyfall: the early orbitals of Sanghelios were wrecked and came crashing down on the surface, and most higher forms of technology ceased to work. Millions were killed in the first day alone, and far more perished in the days, weeks, and months, as the disaster expanded in scope with the collision of asteroids and comets previously held in place. To say that the Sangheilian disaster-relief procedures could not cope was a significant understatement. They had been reduced back to a pre-space age civilization, and their colonies throughout the system were forced to fend for themselves. Without support from Sanghelios, most of the smaller outposts died out, with significant populations maintained only in the best-established colonies.

Though the Sangheili eventually recovered, the Sangheili were permanently affected by the event. Not only did it showcase the power that inert Forerunner god-machines held, but it also saw the disappearance of their most holy artifact. With their belief in science shattered and so much trauma inflicted, many turned back to religion to cope, in particular one belonging to a state that had always opposed reliquary experimentation. By the time the Sangheili returned to space so many centuries later, it was with a conservative, deeply-traditional culture.

Ecology
On Sanghelios, the evolutionary transition from the oceans to land-based life happened relatively quickly due to the frequent flooding of coastal areas, thanks to the planet's high tides. Pseudo-reptilians such as Sangheili are the planet's dominant form of fauna, though other categories (such as mammaloid, insectoid, piscine, and mollusc-like species) exist. The Sangheili's unique quad mandible structure is shared by their entire taxonomic order, though the exact shape these take in different families and genera of species has diverged over time.

A large portion of the plant life on Sanghelios uses retinaldehyde as its photosynthetic pigment rather than chlorophyll, making purple, rather than green, the predominant color of plant life. Although chlorophyll is more common on worlds with complex life, Sanghelios is not unique in this, and purple flora has evolved independently in several biospheres outside that of Sanghelios.

Flora and fauna native to Sanghelios appears to have been transplanted to other worlds in ancient times, resulting in several worlds in the Covenant Sphere housing out-of-place biota descending from Sanghelios life.

Satellites
Sanghelios is encircled at the equator by an orbital ring known as the Silver Girdle. This complex of facilities, which dates back to the Covenant's late antiquity, houses many of the planet's orbital ports, spacedocks and shipyards, along with a number of planetary defenses. Numerous gravitic sky-lifts link to the structure from the surface. Other than the Silver Girdle, Sanghelios is surrounded by a swarm of stations both civilian and military.

It is commonly deduced based on ancient records and even folklore that Sanghelios once had at least two more minor satellites, which were mined out in their early days of space travel.

Suban
Suban is the larger of Sanghelios' two natural satellites and orbits relatively close to the planet. While arid, Suban is habitable to Sangheili, something commonly attributed to Forerunner terraforming in ancient times. Important to Sangheili astronomy and mythology dating back tens of millennia, Suban is the oldest Sangheili space colony, with its first settlers having arrived in the earliest days of Sangheili space travel long prior to the Skyfall. The moon is most notably the source of the Subanese crystals used in Covenant shard weaponry. The production and harvesting of the crystals is monopolized under the Subanite Brotherhood, which holds enormous wealth due to the importance of their export and acts as the de facto government of the moon.

An ongoing cause of concern and public debate regarding Suban is the moon's decaying orbit around Sanghelios. Since the effect was first documented by astronomers in the centuries following the Skyfall, it is commonly attributed to the destruction of the Shard of Heaven, the Forerunner satellite supposedly having maintained the stability of the moon's orbit. However, some prehistoric Sangheili astronomical codices are recorded as having observed the moon's irregular orbit even long prior to the Skyfall. Whatever the case, tidal effects on the planet have been slowly increasing over the last three millennia, and the moon is expected to hit the Roche limit in less than 100,000 years. Without intervention, Suban will be disintegrated by tidal forces, with subsequent apocalyptic tidal effects on Sanghelios rendering the planet uninhabitable long before the moon directly collides with the planet. Various efforts have sought to stabilize Suban's orbit, but even with the Covenant's exotic drive technology, the moon is far too massive for their drives to make a difference. A brute-force solution involving the construction of a vast reaction engine has been proposed, but the scale and cost of such an undertaking has meant the idea has not, as of yet, left the planning stage. Instead, many Sangheili hope salvation will come in the form of an extraordinarily powerful Forerunner engine, and multiple Sanghelian statesmen throughout history have sponsored expeditions to find such a relic. So far, these efforts have not born fruit.

Qikost
A moon largely covered in oceans, Qikost is a notable aquacultural site for the Sangheili and home to various population centers. Its oceans have served as a major source of deuterium for fusion purposes since the Sangheili's early spacefaring days. Although smaller than Suban, Qikost is nonetheless fairly large and has considerable tidal interactions with Sanghelios.