POPULATION: <4 billion (8.135 billion pre-war)
Sanghelios is the homeworld of the Sangheili, seeing their evolution from planet-bound clans to a space-faring race with extrasolar colonies before humanity had built the pyramids. This rapid evolution was due in no small part to the Forerunner artifacts that were left scattered throughout the planet, which the Sangheili revered as relics of the gods. In 938 BCE, the Sangheili met the San'Shyuum, and after a war over the treatment of Forerunner relics, formed an alliance known as the Covenant.
The climate of Sanghelios is considered harsh, and is suggested to have effected the Sangheili's cultural evolution into a feudal system of clans and warriors. This proved beneficial to the San'Shyuum, who used the Sangheili as their loyal protectors while the San'Shyuum took the role of rulers. It also became a significant burden to the Sangheili after the war, as they hadn't had to govern themselves for thousands of years. This, along with the power vacuum that was formed when the Covenant suddenly collapsed, led to a regression of the Sangheili into their old clans and alliances. This time period was known as the Blooding Years, several years of Sangheili civil war, culminating in Thel 'Vadam and his Swords of Sanghelios driving out the last of their major resistance to a central Sanghelios government at Sunaion in 2558.