The Elites
The Sangheili (Latin Macto cognatus, meaning "I glorify my kin") are a saurian species of fierce, proud, strong, agile and intelligent warriors and skilled combat tacticians. Sangheili are named after their home planet, Sanghelios. Known to Humans as Elites, due to their adeptness and skill in combat, the Sangheili formed the military backbone of the Covenant for almost the entirety of its existence. They had a very strong rivalry with the Brutes, which finally became open hatred during the Great Schism, and the conclusion of the Human-Covenant war.
Originally a founding member of the unified conglomerate of races known as the Covenant, they later led the Covenant breakaway faction known as the Covenant Separatists, who sided with the humans against their former allies, due to the fact that the Prophet of Truth and the Prophet of Mercy had ordered the genocide of the entire Sangheili race. This was established after the assassination of the Prophet of Regret at the hands of the "Demon". It was the Sangheili who were ultimately held responsible for the Prophet of Regret's death, and were seen as "unfit" to continue to guard the Prophets. The Prophets sought to achieve this through use of the other Covenant races, primarily the Brutes. The aftermath of the Great Schism formed the bedrock of the Covenant's ultimate collapse.
The Sangheili home world, Sanghelios, had at one point been visited by or had been in contact with the mysterious Forerunners. They revered the objects these "gods" had left, and considered tampering with them a heresy beyond any other. In 938 B.C.E. they encountered the San 'Shyuum (who would later be known as "Prophets" by Humans), a race that also worshiped the Forerunners as Gods, but had utilized Forerunner technology for their own needs, believing that the technology had been left behind by the Gods as gifts for their children. This sparked a war between the two races over how to treat the relics. Even though the Sangheili were much more physically imposing and numerous, they suffered extreme casualties under the sheer power of the Prophets' Dreadnought, a surviving Keyship, and by 876 B.C.E. even the most devout had to admit their survival depended on the reconstitution of their own Forerunner technology. This, of course, led both sides to an agreement that ended with the decommissioning of the Forerunner Dreadnought and a shaky alliance that eventually expanded into the Covenant.
The Prophets helped assuage the Sangheili by informing them of the Great Journey: By seeking out the Holy Rings scattered throughout the galaxy and utilizing them, a Great Journey would begin. This Great Journey would transform believers into godlike beings, while providing salvation for their doomed existence of fighting with the Flood. The Sangheili then became the bodyguards of the physically less imposing Prophets, who would search the galaxy for the Halos. The Writ of Union was drawn up in 852 B.C.E. in order to codify the Covenant. Its first canto clearly outlines the nature of the Covenant from the perspective of the Prophets.
Being one of the few races in the Covenant to achieve space-faring status without outside intervention, the Sangheili interaction with the San 'Shyuum was initially strained due to memories of the recent war. However, the two sides later formed a prosperous relationship, eventually laying the foundation for the modern Covenant hegemony.
During their membership in the Covenant, the Sangheili were technological and societal equals of the San 'Shyuum. However, for a significant period, they found themselves as the dominant military muscle for the entire Covenant structure - hardly surprising given the warlike, feudal nature of their homeworld, Sanghelios. What was surprising was the cyclical stability this feudal society was able to offer. Technological and medical advancement prospered, and the intelligent, aggressive Sangheili were peerless until they encountered the San 'Shyuum. With exchange of goods and ideas, the Sangheili adapted to and eventually embraced the Covenant religion, grafting its tenets into their own belief system.