Well, as quoted here:“Halo 4 is the next blockbuster installment in the iconic franchise that’s shaped entertainment history and defined a decade of gaming. Set in the aftermath of Halo 3, Master Chief returns to confront his own destiny and face an ancient evil that threatens the fate of the entire universe. Halo 4 marks the start of a new trilogy that begins with its release in 2012.” So this mean if we are looking for an enemies to be facing, we need to search to find “Ancient” ones. Well, as most of you know the first contact with the flood EVER, was with the Forerunners at Seaward,"G 617 g"So as we can assume, we could most likely be facing some sort of Flood type enemy, or that is made general inference…Or atleast and old enemy that we never new about.
> Precursors possibly?
Possibly. SPOILER ALERT!!!Because at the end of Cryptum, the thing caged up that had escaped, that the Didact had in his memory, said " I am the last Precursor."
Could be Flood, Forerunner, Precursor, or maybe even sometihng we don’t even know about.