[[Garviel Loken]], newest member of the [[Luna Wolves]] elite [[Mournival]] council, is troubled by recent events on the [[Great Crusade]]. After annihilating the [[Interex]], [[Horus]] orders the [[63rd Expeditionary Fleet]] to crush a rebellion on the conquered world of [[Davin]] at the suggestion of his secretive advisor [[Chaplain]] [[Erebus]] of the [[Word Bearers]] [[Space Marine Legion|Legion]]. Finding that the rebel forces have retreated to the planet's moon and have been corrupted by ancient evil forces from the [[Warp]], Horus leads a desperate assault on the traitors' grounded ship, 'Glory of Terra', where he confronts [[Eugen Temba]], the former Governor of Davin. During his battle with Temba, Horus is mortally wounded by Temba's weapon, the anatheme stolen from the Interex by Erebus. When his [[apothecary|apothecaries]] can do nothing for him, Erebus convinces Mournival members [[Ezekyle Abaddon]] and [[Aximand|'Little' Horus Aximand]] to entrust the Warmaster's life to the humanoid priests of Davin, who secretly worship [[Chaos]]. The [[Mournival]] is split in half as [[Garviel Loken]] and [[Tarik Torgaddon]] oppose this, the latter quitting his membership of the warrior lodge over it. By using ancient rituals learned from agents of Chaos, Erebus invades Horus' mind and attempts to persuade him that the [[Emperor]] plans to betray him to pursue his own glory after the crusade is won. [[Magnus the Red]] uses his powerful psyker abilities to intervene and strips Erebus of his disguise as [[Hastur Sejanus]], but despite his efforts Horus chooses to ally with Chaos. Following his miraculous recovery, Horus leads his legions to war against a newly-discovered peaceful human civilisation alongside the [[World Eaters]] Legion led by their [[Primarch]] [[Angron]] and then begins to hatch his own daring plans of rebellion. His first defiance of the Emperor's will is the murder of the troublesome remembrancers [[Petronella Vivar]] and [[Ignace Karkasy]], before calling his generals to make war on the traitor world [[Isstvan III]].