Basically, if 40k were to ever get an "End Times"(hopefully never) who do you think would actually be the big threat? Chaos? Tyranids? Necrons/C'tan? We seem to already have an alliance forming between Imperium and Eldar and even the Eldar/Dark Eldar/Harlequins are teaming up now. There is A Necron Lord helping them but for selfish reasons and hardly speaks for the whole Necron race so I don't know if that effects their villain status.