He prepares in secret over the next two Terran years to fight a sudden and ruthless galactic civil war. Though his treachery and malign intent remain hidden, he begins his far-reaching conspiracy to suborn those of his brother Primarchs' Space Marine Legions and the other military forces of the Imperium who will aid him and neutralise the threat of those who will not serve him where he can. From this day forth he makes his plans to bring the Imperium to ruin and topple the Emperor from power. He is healed within the ancient Serpent Lodge on the moon but though his life is saved, his soul is forfeit to the Ruinous Powers.

004.M31 Plague Moon Campaign and the Fall of Horus - The Warmaster Horus is grievously wounded while fighting rebels on the moon of Davin.

The Imperial planetary government had collapsed, and the Planetary Governor Vardus Praal, now a turncoat and perhaps become a mutant or psyker, was leading the revolt.

003.M31 Contact Lost with the Istvaan System - Six Terran years after the last official communiqué from Istvaan III, a Death Guard patrol ship on the outer edge of the XIV Legion's fleet picked up an incomplete astrotelepathic transmission which indicated that Istvaan III was in open religious rebellion against the Imperium.

Though Horus turned to the service of Chaos several standard years before the start of the conflict, the Heresy only officially began with the Istvaan III Atrocity. Below is a chronology of the events immediately preceding the start of the interstellar civil war later called the Horus Heresy.

Horus heresy novels timeline