The Wound on Davin

The Setup

Davin had been brought into Compliance during the Crusade. The Davinite serpent-cult — a pre-Imperial religious order whose theology had unsettling overlaps with what the Imperium would later recognise as Chaos worship — had been suppressed but not exterminated. A residual cult survived on Davin's moon.

In 005.M31, Erebus — First Chaplain of the Word Bearers and Lorgar's most trusted operative — arranged for Horus to deploy the Sons of Horus against the moon's xenos remnant. The deployment was framed as a routine sweep. Erebus knew otherwise.

The Anathame

Horus engaged the xenos directly. The weapon used against him was the Anathame — an alien blade with metaphysical properties no Imperial pathologist had previously catalogued. The Anathame's wounds did not heal. Imperial Apothecary intervention failed. The wound began to corrupt Horus's body in ways the Astartes physiology had not been designed to resist.

The Sons of Horus, distraught, accepted Erebus's suggestion to carry Horus to the Davinite serpent-lodge for what Erebus described as last-rite ceremony. The Lodge was, in fact, a Word Bearer trap.

There is a place. I know it. I can take him there. He must go there before he dies.

— Erebus, attributed to Horus's officers

The Visions

Inside the lodge Erebus and the Davinites performed a ritual that placed Horus into a warp-induced fugue state. For an indeterminate period — by external clocks, days; by Horus's internal experience, weeks or years — he received visions designed to break him. Visions of the Emperor becoming a literal god of mortals. Visions of the Primarchs reduced to relics. Visions of betrayal that had not yet happened.

Horus rose from the lodge healed in body. His soul was no longer his own.

Aftermath

The wound on Davin is, by every subsequent Imperial historical reckoning, the single most consequential medical event in the history of the species. The actual ritual that followed was, by Word Bearer theology, the work that the rest of the Heresy executed.

Davin's moon was subsequently destroyed by loyalists; the planet itself was Exterminatus'd in the Scouring. Neither destruction undid what had been done. Erebus survived the Heresy and is, by Imperial intelligence, still active in the Era Indomitus.

Pre-Wound Davin

Davin and its moon were classified as Imperial Cult-Compliant worlds at the beginning of the Crusade era. Davin had been compliant for approximately seven decades when Horus's Crusade fleet returned to the world in 005.M31 to suppress a rebellion by Davin's warrior-priesthood. The rebellion had been triggered by the Davinite priesthood's engagement with what the priesthood classified as "Old Faith" theological arrangements that conflicted with the Imperial Truth's specific institutional requirements.

The Imperial Truth's doctrine on Davin's situation was institutionally clear: the rebellion required institutional Astartes Crusade-era operational response. Horus's decisions to personally lead the response was consistent with the Crusade's broader institutional arrangements. The decision was not suspicious at the time; only in post-Heresy retrospect has the decision been classified as institutionally consequential.

The Specific Wounding Event

Horus's engagement with Davin's moon occurred during the response operation's climax. The Davinite serpent-lodge — a sustained theological-and-operational facility that the priesthood had constructed on the moon's locations — contained an artefact that the lodge's arrangements had been institutionally preserving. The artefact was the Anathame: a daemon-weapon of classified provenance that the Davinite priesthood had been institutionally preserving across approximately three thousand Terran years.

Horus engaged the lodge's arrangements personally during the engagement's operational climax. The lodge's defenders included a senior Davinite priest who wielded the Anathame against Horus during the engagement's conclusion. The Anathame wounded Horus; the specific wound was minor at the time of infliction but consequential across the wound's progression in the subsequent Heresy era.

The Sleeping Sickness

The Anathame's property was that the wound it inflicted produced consequences across the wound's progression. Horus's arrangements following the wounding was a period that the Crusade-era Astartes Apothecary corps had classified as "sleeping sickness" — a sustained psychic-and-biological state during which Horus's consciousness was accessible to Chaos entities that exploited the access to engage Horus's reasoning.

The arrangements for treating the sleeping sickness was the Davinite priesthood's ritual at the serpent-lodge. The ritual completed Horus's transition; the post-ritual Horus was aligned with Chaos in a way that the pre-Anathame Horus had not been. The transition's consequences subsequently produced the Heresy. The Anathame remains, in M42, in classified Imperial Cult custody at a location the Inquisition has not declassified.