I seriously believe the enemy is mainly going to be ONI aka Office on Naval Intelligence.Because they are known to set up at installations like these and known to sacrifice UNSC personnel.Think about it.It definitely shows in the books,games,and minifilms from waypoint.
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The elites and grunts could be not the ones we know it is probably a rebelion against each other and if you seen halo legends elites were against each other so my guess is that those are new type of covenent and probably the new type and the flood might be in it it depends the campaign of the story of the game and the forunners cant be in the game because if u understand halo legends and the books they went extinct after bringing life back on each planet and they remain dead but not their armour [Edited on 04.12.2012 12:05 PM PDT]