Infected Brute Chieftain
From Halopedia, the Halo wiki
This glitch can be done by getting a Flood Infection Form onto a Brute Chieftain. These two do not normally exist in the same area. The only way to get them together is to kill all your allies outside the third tower in the level, while leaving some Flood Carrier Forms alive. Kill the Carriers and let the Infection Forms chase you up into the next snow area. There, kill the War Chieftain and let the Flood take him over.
After being infected, the Chieftain's appearance does not change except for some Flood biomass on his back. However, on rare occasions, and likely on higher difficulties, the Chieftain retains his appearance; there are no signs of Flood infection on his back.
The half-infected Brute may be an old version of combat forms created in-game. The Flood would not totally change the host's body immediately after infection. This idea was cut from the game, possibly because the combat forms looked too similar to the original enemy. The glitch occurs on the level The Covenant because there is a unloading point which removes all of the Flood from the level. When an Infection Form is in the process of transforming a dead Brute into a combat form and the player triggers the unloading point, the Flood Infection Form is physically removed from the level, but ultimately resurrects the Brute. The Brute's AI, having been formerly allied with the Covenant, is now allied with the Flood.