Horus Heresy Overview

Overview

The notes detail the unfolding of the Horus Heresy, the pivotal civil war in the Warhammer 40,000 universe, from the triumphs of the Great Crusade to the betrayal at Isstvan and the opening moves toward the Siege of Terra.

Triumph at Ullanor and Transition of Power

• The Imperium defeats Orks at Ullanor; Horus kills Ork leader Urrlak Urg.
• The Emperor appoints Horus as Warmaster, granting him command over the legions.
• The Luna Wolves are offered a renaming to "Sons of Horus" but Horus initially refuses.
• The Emperor returns to Terra, initiating the webway project and forming the Council of Terra.

Early Conquests and First Signs of Discord

• The 63rd Expeditionary Fleet conquers system 63-19 after failed diplomacy and an assassination of Captain Sejanus.
• Garviel Loken joins the Mournival as an advisor to Horus after showing valor.

The Whisperhead Incident and the Daemon Samus

• Loken’s company encounters vox interference and the name "Samus."
• Battle-brother Jubal becomes possessed, killing his squad before being slain.
• Horus reveals to Loken the existence of warp entities and the need for rational understanding.
• Erebus, Word Bearers chaplain, becomes a new close advisor to Horus.

Warrior Lodges, the Murder Campaign, and Interex Incident

• Warrior lodges foster covert fraternization among Astartes.
• Legions assist Blood Angels against megarachnids on Murder; discovery of weather control structures enables victory.
• Diplomatic incident with the Interex leads to Erebus stealing the Anathame, resulting in war and the destruction of the Interex.

The Davin Betrayal and Horus’ Wounding

• Eugen Temba leads a rebellion on Davin’s moon; Sons of Horus launch an assault.
• Horus is wounded by the Anathame sword, possessed by a daemon in Temba’s corpse.
• Conventional medicine fails; Mournival brings Horus to Davinite shamans for arcane healing.

Horus' Vision and Corruption

• Horus experiences visions in the warp, is manipulated by Erebus and chaos gods.
• Magnus the Red attempts to warn Horus, but chaos ultimately corrupts the Warmaster.
• Horus is healed and adopts the new identity of the Sons of Horus.

Descent into Rebellion and Manipulation of Legions

• Horus secures Auretian STC databases to bribe the Mechanicum.
• Angron and Fulgrim are persuaded to join the rebellion; Fulgrim is corrupted by a daemon blade.
• The Word Bearers spread chaos worship; warrior lodges help identify and purge untrustworthy legionnaires.

Istvaan Atrocities and Loyalist Resistance

• Horus orchestrates the virus-bombing of Istvaan III to eliminate loyalists within his own legions.
• Saul Tarvitz warns loyalists, who mount a doomed last stand; mass slaughter follows.
• The Eisenstein, led by Nathaniel Garro, escapes to warn Terra of Horus’ betrayal.

The Dropsite Massacre at Istvaan V

• Loyalist forces under Ferrus Manus, Corax, and Vulkan assault Horus’ fortified position.
• Traitor reinforcements (Alpha Legion, Iron Warriors, Night Lords, Word Bearers) betray loyalists mid-battle.
• Ferrus Manus is slain by Fulgrim; loyalist forces are decimated.
• Traitors solidify control, marking the true beginning of the civil war.

Aftermath and Strategic Implications

• Survivors attempt to escape; traitors burn the dead and celebrate victory.
• Horus now commands eight legions; three loyalist legions are effectively shattered.
• The path to Terra and the final confrontation is now open.

Decisions

• Horus accepts chaos influence and plans rebellion against the Emperor.
• Horus orders the destruction of loyalist elements at Istvaan III via virus bombing.
• Traitor Primarchs betray Ferrus Manus and loyalists at Istvaan V.

Action Items

• TBD – Nathaniel Garro: Warn the Emperor of Horus' betrayal.
• TBD – Loyalist remnants: Regroup and prepare defenses for the coming civil war.

Key Dates / Deadlines

• Eight-day ritual to heal Horus on Davin.
• Nine-day wait outside Serpent Temple for Horus’ recovery.
• Ten-month campaign against Auretian Technocracy.

Questions / Follow-Ups

• Will Garro reach Terra in time to warn the Emperor?
• Can loyalist legions recover and coordinate a defense before Horus reaches Terra?
• What further legions and factions will join each side as the war expands?