The Emperor consolidates his control over Terra and formally declares victory in the Unification Wars, bringing centuries of conflict to a close. Work intensifies on enforcing the rationalist state ideology of the Imperial Truth across the planet.

Realising that war with the techno-cults of Mars would be a long and costly affair, the Emperor instead brokers an alliance. In return for constructing a mighty war fleet and supplying weapons and machinery for his Legions, the Mechanicum are granted unparalleled autonomy as a client state of the Imperium, and continue to practice their faith in the Omnissiah.

With the furthest reaches of the Sol System firmly under Imperial control, the Emperor and his Legions begin the Great Crusade. The first Explorator fleets leave the system to reclaim surrounding sectors, carrying the hopes of humanity with them.

After decades of disagreement surrounding the employment of psychic powers within the Imperium, the Emperor presides over a grand council to judge their use. After heated debate, he pronounces an edict banning their usage throughout the Imperial military – save for the vital Astropaths and Navigators.

While fighting to quell a rebel uprising, Horus is mortally wounded by a tainted blade. His followers inter him within the Serpent Lodge on Davin where, during a sinister healing ritual, he makes a pact with dark powers. Although the Warmaster emerges from the lodge, he now harbours the festering seed of corruption.

Leman Russ is dispatched to detain Magnus the Red for breaching the Edict of Nikaea, but his orders are manipulated by Horus and instead the Wolf falls upon Prospero with righteous fury. The Thousand Sons are brought near extinction, until a rift in reality hurls them and their Primarch deep into the Warp.

Learning of Horus’ treachery, Magnus attempts to warn the Emperor through a psychic projection, but breaks the Great Plan’s protective wards in the process. Daemons flood into the Webway, forcing the Adeptus Custodes to fight a ten-year war to prevent them pouring into the heart of Terra itself.

Considered by Horus to be his greatest threat, Sanguinius and his Legion are ordered to the Signus system far in the galactic east. There, daemons ambush the Blood Angels, causing horrific casualties until their Primarch decisively defeats the infamous Bloodthirster Ka’bandha in single combat.

The Warmaster reveals the full scale of his treachery at Isstvan III, purging members of his and allied brothers’ Legions still loyal to the Emperor in an apocalyptic virus bomb attack. Despite their best efforts, the frigate Eisenstein escape to bring word to the Imperium, and survivors mount a desperate last stand.

Waylaid fighting Orks on the galactic fringe for five years, the White Scars discover the hand of the Alpha Legion in their misfortunes. After contact with the Imperium is reestablished and word of Horus’ betrayal reaches Jaghatai Khan, the White Scars make full speed for Terra, smashing all Traitor forces they encounter.

The Sons of Horus find themselves blockaded on their homeworld of Cthonia, but are too heavily entrenched to be removed. The Imperial Fists maintain their blockade and blunt several attempts to break free, until an XVI Legion strike force arrives to break the siege and turn the tables on the sons of Dorn.

A Retribution Fleet, dispatched by Rogal Dorn to bring Horus to justice, deploys three loyalist Legions to Isstvan V. After hours of combat, they are betrayed by four fellow Legions and slaughtered amidst the Urgall Depression. Ferrus Manus is killed by Fulgrim in single combat, and Vulkan disappears during the battle.

A vital space lane comes under assault from the Alpha Legion, who through sheer misfortune encounter a loyalist company of the Iron Warriors defending it. The region changes hands many times over the course of five years, leaving the core world of Paramar V little more than a devastated husk.

The Space Wolves, severely depleted since the Burning of Prospero, are ambushed in the dangerous Alaxxes Nebula by the Alpha Legion. Alpharius himself teleports aboard Leman Russ’ ship Hrafnkel, but is rebuffed long enough that a Dark Angels space fort joins the battle and routs the traitor forces.

The Imperial Fists contingent of Dorn’s Retribution Fleet is misdirected by Warp turbulence and ambushed by traitorous Iron Warriors. Dozens of capital ships are lost as hundreds of boarding actions take place on both sides, with the Imperial Fists boarding Perturabo’s flagship and almost securing victory before they are ordered back to Terra.

Primarchs Angron and Lorgar launch a crusade of extermination across the Ultramarines’ realm of Ultramar, burning every world their fleet encounters. Guilliman confronts his brothers at Nuceria and halts their rampage, but not before dark powers possess Angron and transform him into a daemonic avatar of bloodshed.

The Ultramarines and Word Bearers Legions answer a mustering call on Calth, only for the XVII to betray their brothers and butcher them en masse. Although Roboute Guilliman mounts a brave defence, traitor machinations cause the death of the system’s star and bathe the planet’s once-vibrant biosphere in deadly radiation.

Glutted on the sacrifice of Calth to the Dark Powers, a ferocious warp phenomenon dubbed the Ruinstorm cuts large regions of the galaxy off from the Emperor’s guiding light. Three Loyalist legions are stranded far from Terra, unable to assist in the coming defence until it is too late.

Traitor ships assault the Segmentum Solar under the cover of refugee flotillas, bringing the war to Terra’s doorstep. The Alpha Legion comes within striking distance of the Throneworld in the Battle of Pluto, only for Alpharius to meet his death at the hands of Rogal Dorn. Despite this, their command structure remains as strong as ever.

The Dark Angels return to reports of atrocities committed across the Eastern Fringe, revealed to be the work of the Night Lords. The vengeful conflict that follows lays waste to hundreds of Imperial territories, but ends with Konrad Curze and many of his senior commanders presumed captured at the hands of the Lion.

An impregnable fortress created by Primarchs Perturabo and Fulgrim comes under futile attack by an Imperial Army cohort, who are run down and butchered by the Emperor’s Children. The defenders then turn to find the Raven Guard occupying their vacated fortress, who swiftly turn the powerful defensive weapons on their careless former brothers.

Baal, home world of the Blood Angels, becomes a sanctuary for Space Marines fleeing Horus’ rebellion. Fearing the Shattered Legions will reorganise and pose a threat to his plans, the Warmaster besieges the planet, although every attempt to conquer Baal is rebuffed with a heavy toll of Traitor casualties.

The Iron Warriors launch a daring void assault against the orbital hives of Vanaheim, siezing them from the Mechanicum. Space Wolves and other Loyalist forces attempt to reclaim the facilities, with countless Drop Pod assaults falling like hail on the hives, but three successive counter-assaults fail to achieve their goals.

The vital Forge Worlds of Anvillus erupt into a war of mutual annihilation. Strangely, the Magi lords reject offers of alliance from both Loyalist and Traitor causes, instead destroying any who attempt to intervene. No onlookers are able to determine what the opposing factions are warring over, or why the conflict began.

Dozens of Traitor fleets attack the Inwit Cluster, hungry for the prize of the Imperial Fists’ homeworld. The siege increases steadily in intensity for four years before the Warmaster’s direct assault on Terra draws Traitor forces away, leaving the Cluster a string of burning, wasted worlds.

The Traitor cause commits an enormous invasion force to the Knight World of Molech, including four Titan Legios. At a critical moment, the Knight House Devine reveals its allegiance to Horus, turning the tide against the Loyalists in a treacherous firestorm that spells doom for the Imperial defenders.

A belt of highly concentrated Forge Worlds is wracked by a crisis of faith over the Emperor’s legitimacy as their Omnissiah, sparking a wave of religious civil wars. Even the intervention of the Dark Angels cannot resolve the spiralling disaster, and battles between dozens of Forge Worlds continue far beyond the Horus Heresy.

An abatement of the Ruinstorm allows Rogal Dorn to muster many formerly stranded forces at Beta-Garmon, forming the largest concentration of Loyalist forces in the galaxy. Horus has no choice but to respond, and the resulting monumental clash sees billions of lives lost and the might of the Collegia Titanica all-but broken.

The White Scars and Emperor’s Children conduct a high-speed duel across the last critical space lines connecting Terra to the northern Imperium. The arrival of a massive Dark Mechanicum armada turns the tide, forcing the White Scars out of their element and ultimately ejecting the Loyalists from Lorin Alpha.

Freed from their isolation by the Ruinstorm’s disappearance, the Dark Angels wreak havoc across the southern Imperium in a campaign of revenge against any loyal to Horus. A trail of blackened, scorched worlds is left in their wake, including the homeworlds of the Emperor’s Children and Death Guard Legions.

The Space Wolves attempt an ill-counselled assassination against Warmaster Horus, and are scattered into the Yarant system where they make a desperate last stand. Leman Russ’ many enemies press his Legion to near-extinction, and the Space Wolves are only saved by the unexpected intervention of Corvus Corax.

Despite the apparent death of their Primarch, the Alpha Legion defy Horus’ orders and set out to hinder Loyalists approaching from the galactic east. Roboute Guilliman’s battle fleet is forced to intervene to prevent further losses of vital reinforcements, critically delaying their arrival at Terra in time for the upcoming siege.

Drop pods of the Death Guard land upon Terra’s soil, the first of the Traitors to deploy. Many of the warriors that emerge are twisted mockeries of Legionaries, their armour cracked and decaying, their forms bloated by the corrupting power of the æther. The Traitors move to capture the outer defences of the Daylight Wall; the Khan and several thousand of his White Scars, accompanied by Sanguinius, Primarch of the Blood Angels, sally forth to oppose the landing. The Death Guard’s advance is halted, and the Loyalists gain valuable data on the siege engines being unloaded from Traitor Ark Mechanicus landing further from the walls.

The Daylight section of the Eternity Wall is subjected to a coordinated multi-front Traitor assault. Corrupted engines built by the Mechanicum bombard the walls while the Death Guard march on the outer defences. The sky weeps blood as the Traitors enact fell rituals, and Night Lords assault companies are the first Traitors to set foot upon the walls. As the battle rages, a falling star slams into the earth before Helios Gate; from the resulting crater emerges the Primarch Angron, his form twisted by the powers of the Warp. The Red Angel massacres thousands, and the outer defences fall to the Death Guard, but the Night Lords are driven from the wall, and Helios Gate holds.

Led by Lord Seneschal Fafnir Rann of the Imperial Fists, the Loyalists defend the Lion’s Gate Spaceport for weeks against a massive force led by the Iron Warriors, commanded by Perturabo. After a series of feints, the Traitors launch an overwhelming assault, attacking from above through the spaceport’s orbital elevators and over its walls. The defences of the spaceport are overrun, and only the arrival of reinforcements commanded by Sigismund, First Captain of the Imperial Fists, and then by Rogal Dorn enables the Loyalist survivors to retreat. Dorn and Perturabo meet face to face but refuse to cross blades. As the Loyalists withdraw from the spaceport, tears in reality begin to open as the Emperor’s psychic shield wavers, allowing entities drawn from the Warp to begin manifesting on Terra.

With the loss of the Lion’s Gate Spaceport, the Traitors begin to unload heavier armour and titans through its orbital towers. The next phase of the siege begins with Dorn identifying four strategic locations that the Traitors must next target: Gorgon Bar, Colossi Gate, the Saturnine Gate and the Eternity Wall Spaceport. Declaring it impossible to retain possession of all four locations, Dorn deems the spaceport the least important, owing to the Traitors already having gained orbital control. The remaining three locations receive an influx of defenders, with a Primarch assigned to each. Dorn himself assumes command of the defence of the Saturnine Gate, while Sanguinius leads at Gorgon Bar and the Khan makes his stand at the Colossi Gate.

Gorgon Bar is assaulted by the Iron Warriors, who unleash unrelenting bombardments against the defenders as cover for the advance of Legio Vulpa. The Titan Legion is routed by Sanguinius, who, by his own hand, slays a Warlord titan and inspires the defenders to push forward. The Iron Warriors are driven from the towers of the Fourth Circuit, strengthening the Loyalists’ position.

True to his nature, the Khan refuses to lead a static defence at the Colossi Gate. With First Captain Raldoron of the Blood Angels and Captain-General Valdor of the Legio Custodes by his side, the Khan leads his Legion in a series of sorties against the Traitor positions, inflicting massive casualties upon the besieging Death Guard. Only the intervention of Magnus and his Thousand Sons prevents a total rout, with the sorceries of the XVth Legion and manifesting Daemons forcing the White Scars to abandon their assault and retreat behind the gate.

Rogal Dorn lays a trap at the Saturnine Gate, presuming Perturabo is aware of a flaw in the edifice’s construction. The Sons of Horus, led by First Captain Abaddon, launch a subterranean assault while the Emperor’s Children attack the gate itself. Loyalist kill teams ambush the Sons of Horus as they emerge from the ground, slaughtering thousands and forcing them to withdraw; of the Mournival, all but Abaddon are slain. Dorn leads the defence against the IIIrd Legion, duelling with Fulgrim. When news reaches Fulgrim that the Sons of Horus’ assault has failed, he withdraws the bulk of his Legion from the front lines, refusing to commit en masse to further assaults.

The Eternity Wall Spaceport falls to a Traitor assault. Commanded by Captain Camba Diaz of the Imperial Fists, the Loyalists refuse to surrender to Angron, remaining steadfast in the face of inevitable death. The World Eaters, supported by Legio Audax, turn the spaceport into a charnel house, securing another landing zone for the Traitors.

The Warmaster, commanding from his throne room aboard the Vengeful Spirit, orders the Death Guard to assume the Iron Warriors’ position in the Lion’s Gate Spaceport. Perturabo is ordered to disperse his warriors across the warfront; he refuses onthe grounds that the war holds no purpose save blood for the sake of blood, and the hubris of both Emperor and Warmaster. Perturabo orders the IVth Legion to withdraw from Terra and the Sol System, as some ships fire upon the departing IVth and others scramble to fill the gaping hole left in the orbital blockade.

Corswain and his fleet arrive on the edge of the Segmentum Solar, linking up with the hidden survivors of the Solar Fleet led by Admiral Su-Kassen. Determined to land on Terra, the Dark Angels bide their time before pushing for the Throneworld following the Iron Warriors’ departure from the Traitor blockade. The Emperor’s battle-carrier, Imperator Somnium, is sacrificed to ensure the Dark Angels break through with Corswain and 10,000 Legionaries landing upon the Hollow Mountain, driving out the Emperor’s Children and Daemons that occupied the Astronomican. Corswain intends to reignite the beacon to guide the approaching Ultramarines and Dark Angels fleets to Terra. News of Corswain’s arrival reaches the defenders of the Imperial Palace, who initially believe the Lion has finally arrived to relieve Terra. The absence of most of the First Legion is discovered shortly before all communication between Dorn and those outside the Sanctum Imperialis is cut off.

A tide of fallen titans, scavenged from Beta-Garmon and animated by sorcery, assails the Loyalist defences, the full strength of the Legio Mortis marching behind them. Legio Ignatum, supported by the remnants of over a dozen other Titan Legions, including Legio Solaria and Legio Atarus, marches out to prevent the Traitors from reaching the Mercury Wall. Weeks of fighting follow, and though dozens of Traitor engines fall, Legio Mortis break through and breach the Mercury Wall.

The Sanctum Imperialis lies open.

A pall of hopelessness, conjured up by the Primarch Mortarion, falls over the defenders, most accepting what they view as inevitable defeat. The Khan refuses such a notion, mustering his Legion and thousands of Imperialis Auxilia armoured units to sally forth to retake the Lion’s Gate Spaceport. The sortie carves through the Traitors’ lines, allowing the White Scars to push into the spaceport proper, clashing with the Death Guard who defend it. Mortarion and Jaghatai cross blades; the Death Guard Primarch is seemingly killed, with the Khan rendered catatonic from his wounds, spending the remainder of the siege at the very precipice of death. The spaceport falls to the White Scars, who assume control of its defences and reactivate its orbital cannons, a pocket of Loyalist resistance amongst a sea of Traitors.

The Traitors move to seize the Eternity Gate to gain complete access to the Sanctum Imperialis. Sanguinius leads the ad hoc force of defenders atop the Delphic Battlements. The battle swiftly descends into a brutal melee once the World Eaters reach the wall. In the skies above, Sanguinius clashes with the Daemon Ka’Bandha, breaking its back and casting it back to the Warp. Soon after, Angron hunts Sanguinius. In the resulting duel, the Great Angel is severely wounded, while Angron is laid low. The Eternity Gate is sealed shut from within, but the inner walls are breached elsewhere in several places.

The final days of the siege approach.

The Emperor learns that the shields of the Vengeful Spirit have been lowered, leaving it vulnerable. Malcador ascends to the Golden Throne, an act that the Regent of Terra knows will spell his death, to allow the Emperor to lead an assault against the Warmaster. Accompanied by Rogal Dorn, Sanguinius, Valdor and their warriors, the Emperor teleports aboard the Vengeful Spirit. The Traitors lie in wait for the Loyalists, and a deadly boarding action takes place during which the full corruption of the Vengeful Spirit is revealed, with Daemons and reality-warping sorceries unleashed upon the boarders. The Emperor pushes through, determined to meet his turncoat son in combat.

The clash that follows is one shrouded in half-truths and myth. What is known is that Sanguinius is slain by the Warmaster himself, while the duel between the Emperor and Horus leaves the former broken and the latter dead. Rogal Dorn discovers the dying Emperor and returns him to Terra, interring him in the Golden Throne to be sustained upon the precipice of death. The fall of the Warmaster shatters the loose alliance of Traitors; some withdraw from the Throneworld while others fall into infighting or fight the Loyalists to the bitter end. In the following weeks, purge teams hunt down those Traitors that remain. At the same time, the arrival of the Space Wolves, Ultramarines and Dark Angels sees the formation of fleets that begin the Great Scouring – a full accounting of which is a tale for another tome entirely.