Instead of playing as Master Chief you play as an elite. You have other elites on your side, fight spartans and marines. Note: this is for Halo PC (Combat Evolved) not Halo CE( Custom Edition )
So now you can play as an elite on the level assault on the control room. All marines are replaced with elites. You fight spartans and marines. This map might be very hard but trust me, I've completed the mod without devmode.