A mission to return the moon Davin to compliance goes horribly wrong, leaving the Warmaster at death's door. The Sons of Horus try everything to save him, but will their efforts damn them?
- Alas, Poor Villain: Horus weeps bitterly over the death of Eugan Temba, who was a friend of his before his corruption by Chaos.
- Banishing Ritual: Euphrati Keeler banishes a daemon back to the Warp by invoking the name of the Emperor and brandishing an aquila pendant at it.
- The Chessmaster: It's here that we see how much of a manipulator Erebus is - he arranges a situation where he initially appears untrustworthy so that he can beg forgiveness and stoke Horus' ego, so that the Warmaster will be more pliable to his other points. Karkasy actually admires how well he played the part.
- Crisis of Faith: Kyril Sindermann loses faith in the secular Imperial Truth after accidentally summoning a daemon and seeing Euphrati banish it. This is ultimately what brings him around to the Lectitio Divinitatus.
- Defector from Decadence: Torgaddon quits the warrior lodge in anger when they tell him that Loken is to be made the fall guy for the massacre on the embarkation deck.
- Dramatic Irony: Part of the vision that Horus sees in the Serpent Lodge - which leads him to rebel against the Emperor - is a colossal church where the Emperor, Sanguinius, Leman Russ, Rogal Dorn, Ferrus Manus, Lion El'Jonson, Guiliman, Corax, Vulkan, and the Khan are honored, but the rest of the Primarchs are nowhere to be seen. Readers, however, will recognize that this is a portion of the Ecclesiarchy, meaning the future that Horus saw is the one he will create.
- End of an Era: The final portion of the novel is titled Crusade's End, and the title is fitting, as it details the end of the Great Crusade - Horus is no longer trying to expand the Imperium, but instead is readying his forces to conquer it.
- Face–Heel Turn: Horus officially makes his in this book, declaring war on the Emperor and Imperium in order to stop what he believes is his father's quest for godhood.
- Good Angel, Bad Angel: Within Horus' vision, Erebus-as-Sejanus is the Bad Angel, trying to edge Horus toward rebellion. Magnus the Red, meanwhile, is the Good Angel, seeing Erebus for who he is and urging Horus not to turn. Not only will Magnus fail, but what he does afterwards will damn his whole Legion.
- Manipulative Bastard: Erebus. He corrupts Eugan Temba and gives him the anathame, then brings the Sons of Horus to Davin and expertly talks Horus into facing the man personally with a few well-chosen turns of phrase that bruise Horus's pride and stroke his ego. Then he manipulates the Mournival into taking the Warmaster to the Serpent Lodge for healing by playing on their anger, fear, and filial love for their gene-father.
- Never Suicide: Maggard murders Ignace Karkasy and makes it look like a suicide.
- Pride: A major theme.
- Part of why Horus falls is because he can't stand the idea of a world where he's no longer a glorious conqueror who is constantly celebrating new victories but "merely" the keeper of galactic peace.
- Even sympathetic Astartes like Loken find the idea of Astartes being held responsible for their actions by mere mortals to be unthinkable.
- Magnus is warned that using sorcery to contact the Emperor might not end well, but he won't reconsider since he sees it as a chance to prove the value of sorcery and therefore by extension prove that he was right all along to study it.
- The Purge: Hektor Varvarus and Ignace Karkasy are murdered for demanding that the Sons of Horus be held to account for their killing of civilians. This marks the point at which Horus begins to remove anyone who won't support his rebellion.
- Self-Defense Ruse: Horus murders the emissary from the Auretian Technocracy and justifies it by claiming that the man had a weapon and was going to kill him. Loken doesn't buy it.
- Self-Fulfilling Prophecy: Horus has a vision of the Imperium as a superstitious, brutal, regressive theocracy where he and half of his brothers have been forgotten while the Emperor is venerated as a god. Unbeknownst to him, his efforts to stop this future are what will bring it about.
- She Knows Too Much: Horus murders Petronella Vivar after revealing too much of himself to her on his sickbed.
- Undying Loyalty: Petronella Vivar's bodyguard pledges his to Horus after the fighting on Davin.
- Unfriendly Fire: Hektor Varvarus is murdered by a shot from an Astartes bolter during the war with the Auretian Technocracy.
- Vision Quest: Erebus leads Horus on one while the Warmaster lies on death's door in the Serpent Lodge.
- What the Hell, Hero?: The Mournival and the other Sons of Horus deeply damage their standing when they violently trample a crowd of people in their haste to get an injured Horus to the Apothecaries, killing a score of innocents in the process. Loken somewhat feebly argues that they were driven by desperation, but Karkasy roughly tells him that Space Marines are venerated over regular men specifically because they're supposed to be above such human frailties.
- Wound That Will Not Heal: Temba uses the anathame to inflict a minor injury on Horus. Ordinarily, his transhuman physiology would have healed such a wound almost instantly, but the anathame's sorcerous nature means that it festers and brings him to the brink of death despite the best efforts of the Legion's Apothecaries.
- Zombie Apocalypse: Davin's moon is overrun by plague zombies.
