ThePrometheans were the highest and most respected Warrior-Servants in the Forerunner empire. During the Human-Forerunner War, they were led in battle against the Humans by the Ur-Didact, the supreme commander of the Forerunner military and a Promethean as well. Their primary base of operations was the Shield World known as Requiem.

During the Forerunner-Flood War, the Forerunners were converted into digital intelligences known as Knights by a powerful Forerunner machine called the Composer. The flood could only attack biological lifeforms, so the Prometheans made very effective soldiers. Due to a lack of Prometheans to convert, the Ur-Didact also composed a large amount of Humans. He was later imprisoned for this, on Requiem, with the Prometheans ordered to prevent his awakening. The Prometheans and Ur-Didact survived the activation of the Halo Array, and eventually became a new threat to the UNSC.

The mechanical Prometheans on Requiem had various forms and classes. Knights were leader units and the main fighting force. Watchers were small support drones that could be launched from a Knights shell and could provide an energy shield or revive fallen Prometheans. Crawlers were small pack-oriented wolf-like creatures designed to overwhelm opposition from different directions. The Prometheans also used several weapons, including the Boltshot (pistol), Scattershot (shotgun), Lightrifle (rifle), Binary Rifle (sniper), Suppressor (machine gun), Incineration Cannon, and Pulse Grenade. A flyable Promethean vehicle, the Phaeton, is also appearing in Halo 5.

Prometheans first appear in Halo 4, and are the main enemy along with the Covenant. The plot of Halo 4 focuses mainly on the Master Chief trying to stop the Ur-Didact, who was awakened accidentally, from using the Composer on all of Earth. Prometheans have also been confirmed to be returning in Halo 5, and some early gameplay containing them has already been seen.