Yeah, guys actually what we are talking about is really complex. It's weird how we saw the dark Aeons in X2. But like the other guy said, in the omega runins you fight ultima weapon. After the fight Auron says "That was just Omegas Shadow, wait until we fight the real thing" The dark aeons we see in 10-2 i think are just shadows of their past beings. Well what i think is the fayth came back for awhile but not all of them. Mabye it was just the bad part. Like just their combat part of their beings. Mabye thats why Yuna couldn't summon them herself because she is pure at heart. After all they had shuyins vengful spirit backing them up. Mabye Shuyins Spirit well his Hate really Corrupted the Aeons themselfs. But in FFX i really don't know how dark Aeons are possible? Unless their was some sort of evil driving them. Like in the International version of FFX i'd say it was Panice's ( I think that is his name) hate driving them. Like in FFX2 except its shuyins hate of his vengeful spirit.