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During a mission on the planet Davin, Horus was wounded, and required downtime to heal. During his downtime, he was inducted into a "warrior lodge." Although the ceremony was intended to make him a "brother" rather than an outsider, the lodge was rife with Chaos taint, and it affected Horus greatly. He introduced the ideal to the Chapters under his direct command, and then it spread to half of the Legions. Horus also introduced the concept of desposing the Emperor and installing himself, a strong leader willing to take the fight to the enemies of Mankind.
The Emperor sent Horus to Istvaan III to suppress a rebellion. Horus dropped a bioweapon into the atmosphere, which killed the 12 billion inhabitants in seconds; the psychic shockwave was said to be louder than the Astronomican.
Nine Space Marine Chapters declared their allegiance to Horus, and the remaining nine fought them across the Empire in the horrific Horus Heresy. As the civil war drew to a close, the Traitor Legions beat at the defences of Holy Terra.
Horus ordered the deflector shields of his flagship dropped, so the Emperor and his bodyguards could teleport onto the ship, possibly out of some vestige of honour that remained in Horus's body. Sanguinius, Primarch of the Blood Angels, appeared before him, and Horus offered him riches and power if he denounced his allegiance to the "false" Emperor. Sanguinius refused and attacked. Horus quickly killed him, and was still standing over his broken body when the Emperor and Rogal Dorn, Primarch of the Imperial Fists, entered the chamber.
The Emperor fought Horus, power sword against lightning claw, with brutal blows that would eviscerate any mortal man dozens of times over. When the Emperor dropped to his knees, Horus prepared to kill him, but he hesitated. Perhaps Horus remembered that the Emperor was his friend, and a father to him, but whatever caused it, it was enough for the powers of Chaos to take their power away from him, allowing the Emperor to deliver the fatal blow.
Abaddon the Despoiler, Chapter Master of the Black Legion (formerly Sons of Horus), found Horus's nearly-dead body after Rogal Dorn left the ship with the bodies of Sanguinius and the Emperor. Before he died, Horus told Abaddon to "let the galaxy burn." Warhammer 40,000