Sangheili

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 skill in combat, the Sangheili formed the military backbone of the Covenant Empire 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.

(Modified from Halo Wiki)