 In the meantime Horus' rebellion erupts openly and the Loyalists suffer catastrophic losses in the Dropsite Massacre (Book 5).Sanguinius and his Legion, now fully aware of the great betrayal and the reality of Chaos, race to the defense of Terra and the Emperor while buffeted by unprecedented navigational difficulties; it is hinted that these are related to the action in the Ultramar system of Calth (Books 19 and 24).22.