After the civil war called "The Horus Heresy" ended and the legions who had joined Horus were defeated, they fled towards The Eye of Terror to escape into The Warp.
They fled towards The Eye of Terror. This was to escape into The Warp. This was after the civil war called "The Horus Heresy" ended. This was after the legions. These legions had joined. Horus were defeated.