The Prometheans seem to be one of the following: 1. A Forerunner belonging to the Warrior caste, which we have not seen before. 2. A form of mechanical memory storage inside of a combat worthy vessel, similar to your war sphinx constructs. 3. Forerunner combat AI's, similar to sentinels, but with a much higher capacity for intelligence, like the monitors, or the Mendicant/Offensive Biases. 4. A new race, that has a similar history to the Covenant, having found a Forerunner Artifact(s) (or possibly a Forerunner itself, if the Didact is really tagging along now) and decided to worship it/them, but in a completely different way. Or perhaps a devolved form of the Forerunners, that while having enough to survive, had limited gene pools resulting in devolution. 5. It may be any of 1-3, but also perhaps a separatist faction of the Forerunners, similar to the Elite forces in H3, or the Insurrectionists However, seeing as there isn't much canon on these guys yet, there isn't much else we can do but speculate till we get more info, and then speculate more, till the games are complete, but we have unfinished story lines, so we speculate till we have the fan accepted conclusions :) which we will probably never achieve seeing as it is human nature to disagree and -blam!- at each other for the sake of it. Not that there is anything wrong with that. It's kinda fun actually :)