User blog comment:DirgeOfCerberus111/The end is nigh!!! Now what?/@comment-3562230-20110823141919

Come to think of it. If the Astronomicon failed, I don't believe the Tyranids would be able to find Terra. So, the Hive Mind would probably launch attacks all over the galaxy until he was close enough to feel it.

Necrons would be a huge threat because the Imperium has never fought a truly large Necron force before. If the Necrons did purge the galaxy of most human life they would defeat Chaos, who can no longer feed on human emotion. If Abaddon set up a dark empire on Terra he would be hard pressed to stop the Necrons.