Agree with the comments on Mirror Monsters, although Lords/Unknown really struck me as being particularly threatening when I was watching Agito. Difficult as hell to kill and gruesomely imaginative with some of the ways they kill.

Definitely went with Inves and the Over Lords. They are basically like the Flood from Halo. An infection without a cure that basically corrupt your DNA to make you one of them. It's a frightnening thought to think of.

What about the Greeed?