Sanghelios

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

Primeval Period
This period of rapid technological progress 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 caused the Forerunner station 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.

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.