"The heresy takes root"
- —The book's tag-line
The Great Crusade that has taken humanity into the stars continues. The Emperor of Mankind has handed the reins of command to his favoured son, the Warmaster Horus. Yet all is not well in the armies of the Imperium. Horus is still battling against the jealousy and resentment of his brother Primarchs and, when he is injured in combat on the planet Davin, he must also battle his inner daemons. With all the temptations that Chaos has to offer, can the weakened Horus resist?
In this book the Warmaster Horus' Legion of Space Marines, the Sons of Horus, are told by Erebus, the First Chaplain of the Word Bearers Legion, that the planet Davin has become a non-Compliant world and is in rebellion against the Imperium of Man. Furthermore, Horus is told that Davin's Planetary Governor, Eugen Temba, has been spouting traitorous remarks about the Emperor of Mankind and Horus himself. Horus becomes livid at this statement, and orders an Astartes speartip to assault Temba's forces.
Horus despatches the Sons of Horus to Davin 3, a moon of Davin, with a heavy contingent of manpower as well as an Imperial Titan to support them. Upon making landfall, the Space Marines find that the moon has become a decaying, swampy version of its former self, with a yellow misty haze making vision difficult. While the Legion is marching double-time towards Temba's location, his fallen Imperial Battleship, undead Plague Zombies, the remains of Temba's garrison of Imperial Army troops, begin to rise and attack from the ground beneath the Space Marines' feet.
With his men being progressively slowed down by the undead, the Warmaster treads on alone to the command deck of Temba's badly damaged flagship. Upon reaching the command deck, he finds that Eugan Temba no longer resembles himself, for he has been mutated by the Warp through his devotion and worship of the Chaos God Nurgh-leth (Nurgle). Temba and Horus engage in combat, and the Warmaster is wounded by a Nurgle-corrupted xenos blade called an Anathame that Temba is wielding.
This weapon of Kinebrach design had been stolen by Erebus from the Interex's Hall of Devices. It was a specially crafted Chaotic artefact that carried a Nurgle-spawned toxin capable of defeating even the immune system of a genetically-engineered Primarch. Though Horus defeated Temba, the Anathame injected its toxin into Horus' bloodstream, unleashing a crippling illness within the Warmaster's body that Imperial science could not cure.
With Horus on the brink of death, Erebus convinced the Sons of Horus' Mournival that there only option to save him was to take the wounded Warmaster to the Temple of the Serpent Lodge on Davin, a temple dedicated to Chaos and run by a sect of Chaos Sorcerers. While in the temple all of the members of the Mournival were locked out save for the Warmaster. A priestess of the Serpent Lodge slit the neck of Erebus in order to use his life energy to begin the process of healing Horus as well as send his soul into the Warp to deceive Horus into believing that the Emperor of Mankind was betraying all the goals for which the Great Crusade had been fought.
Magnus the Red, the Primarch of the Thousand Sons Legion, used his own command of forbidden psychic sorcery to contact and warn Horus within the Immaterium that great danger lay before him and that he was being deceived before Erebus could meet with him in the induced vision state. Through the illegal use of his sorcerous powers Magnus and his Astartes took the form of a pack of wolves, and reminded Horus of who he was and what that meant to the Imperium and Mankind as a whole. Erebus appeared within Horus' vision as a slain member of Horus' Mournival named Hastur Sejanus shortly after Magnus' image dissipated. Horus was then falsely shown by the Chaos Gods a vision of the far future in which the Emperor of Mankind and the 9 Loyalist Primarchs were now worshipped as a God and his saints, respectively, by the Imperial Cult. The Chaos Gods' vision made no mention of how such a future had come to pass only because of the events of the Horus Heresy that Horus himself would set into motion if he turned against his father. Erebus whispered to a disbelieving Horus that the Great Crusade had been staged simply to allow the Emperor to become a God despite all the teachings of the Imperial Truth and that he planned to discard Horus and all his brother Primarchs like dulled weapons once his ambitions were realised.
Magnus returned to the vision once more, and confronted Erebus directly, revealing that Horus had not been talking to Hastur Sejanus all along, but the deceitful First Chaplain of the Word Bearers. Erebus offered forth explanations and accusations, but Horus was not swayed by either Magnus or Erebus and declared that he had made his decision to accept the offer of the Ruinous Powers and stake his own claim for rulership of the galaxy. This was the turning point that officially marked the beginning of the Horus Heresy, as Horus fell under the influence of Chaos due to his own pride and overweening ambition.
Following his miraculous recovery, Horus leads his Legions to war against a newly-discovered peaceful human civilisation, the Auretian Technocracy and then begins to hatch his own daring plans of rebellion against Terra.
To mark the start of his betrayal of the Emperor, Horus performs three murders in quick succession: Lord Commander Hektor Varvaras of the Imperial Army is shot by an Astartes bolter, Horus' own Remembrancer Petronella Vivar is personally killed by the Warmaster so that she can never reveal what she has learned about him to the public, and the Remembrancer Ignace Karkasy is killed by Petronella's bodyguard Maggard, who betrayed her as well to serve Horus. It is these final acts which firmly set Captain Garviel Loken and Captain Tarik Torgaddon on a collision course against their own Legion. Horus then calls all the commanders of his 63rd Expeditionary Fleet to make clear their next target, a place of dark destiny, the world of Istvaan III.
- False Gods (Novel) by Graham McNeill