C'tan...I've never used one and actually have it cause any havoc. Against IG, Nightbringer was burned up and died (last mistake I make!). Anyway, it all depends. Against a Hord army, you use Nightbringer (except in the case of an IG horde army; in that case, use Deceiver). To be quite honest, I like Deceiver better. The psychological characters have always been more appealing to me. Now, fluff wise, OUTSIDER RULES!