Prometheans

The Prometheans are a race of part robotic, part organic defense AIs that patrol the many layers of the artificial planet known as Requiem. They were created by Forerunners, specifically the Didact, by altering the genes of early Humans.

They were used by the Forerunners to imprison the Didact, and they had alleigences with no-one until a Reclaimer, Jorgey, crashed onto Requiem. Jorgey used the Prometheans, as well as Covenant slaves, to build a massive remake of Reach out of stray asteroids, junk and destroyed starships. This was the prelude to Jorgey's Master Plan.

When Johnny landed on Requiem he awoke the Didact, who took control of all the Prometheans in the area. This didn't matter to Jorgey since the junk Reach was complete, but he did have to fight off the Prometheans who revolted and fought a war against his Covenant forces. Jorgey lured Johnny to the Temple of Requiem to bully him, and in the meantime the Didact and his Prometheans were being bullied themselves. Despite an alliance between the remainder of the Covenant and the Prometheans, the Didact was having enough trouble fighting the Humans and several angry Engineers were at his doorstep. He fled Requiem to retrieve the Composer, since Jorgey had bullied his original Composer off him.

At this point Johnny realised the true origins of the Prometheans, and travelled to the Didact's ship to confront him. He eventually killed the Didact and restored the Promethean's aggresive neutrality.

There are several variations of Promethean, the known ones are listed here

Promethean Knight
Promethean Knights are large bipedal constructs who wield swords and assorted projectile weapons. They are tactical geniuses who normally lead other factions of Prometheans into battle. Each Promethean Knigh has a mutated human skull implanted in its head to act as a case for its AI. It can be assumed that only Promethean Knights were humans once, and the other Prometheans are simply advanced AIs, but this is simply speculation and is far from confirmed.

Promethean Crawler
These irritating little buggers can crawl over walls and relentlessly purue their targets. Despite this, they can be killed with one shot of any weapon and sometimes miss their attacks completely, bouncing bullets off walls straight into their eyes and blinding themselves, then running blindly off the edge of an abyss. Smart.

Promethean Watcher
No, they are not paedophiles. Well, we don't know that, but its not relevant. What they are is a twisted rip off of the Sentinels, all they've done is remove their Cillit Bang launchers and stuck tennis rackets to the side. They fly around and shoot stuff, but are vulnerable to everything, even each other. They can sometimes be seen being pooed out of the back of a Promethean Knight. Creepy.

There is a charity that was set up to save the Prometheans from their fate of eternal slavery, called Help the Prometheans. It is considered an anti-Didact charity, and so it gets billions of donations every week.