Great Scouring

In the Great Scouring the Imperium of Man blew up the foul traitor forces that were left over from the Horus Heresy, herding them into the Eye of Terror instead of finishing them off.

It is a period of the timeline that doesn't have a lot of fluff for it at the moment as little was known about it before the Horus Heresy series of books began and the writers are still working their way up the siege of terra at the moment. Hopefully once Dan Abnett has written the epic that is the Emperor bitch-blasting Horus into a million soul fragments we will see a lot of writing about this period of the history.