This is one of three summaries of the Horus Heresy, a long-running series set in the world of the tabletop wargame Warhammer 40,000.

There are alternate summaries below:

The Horus Heresy in 100 Words

What Actually Happens in the Horus Heresy?

To learn more about what’s going on, consider reading Summarising The Horus Heresy, or explore the broader set of Horus Heresy essays.

The Horus Heresy is a fall of Lucifer tale. The Emperor wages a galactic crusade to restore scattered humanity under one banner. His favoured son, Horus, loses faith in the cause, and commits himself to gods of chaos, who lurk in warp space. The subsequent civil war tears the galaxy apart. There are eighteen legions led by primarchs – in our religious analogy, archangels – and half of them fall, taking their legions with them.

The war plays out in three parts. The first part sets the stage. We see how Horus and the other traitor primarchs fell to Chaos, and in the first major confrontation on Isstvan V, the traitors shatter three loyalist legions and kill one of the loyalist primarchs, Ferrus Manus. Allies and supporting forces are drawn up on both sides, including the Mechanicum techno-artisans and the various assassin guilds. Elsewhere, Magnus and the Thousand Sons have been practicing outlawed sorcery. Magnus wants to use his powers for good, but ends up causing a bunch of problems. The loyalist Space Wolves punish him by razing his homeworld. Magnus later becomes the final member of the traitor legions.

The second part is the Ultramar campaign. The loyalist primarch Roboute Guilliman, leader of the Ultramarine legion, rules over the 500 Worlds of Ultramar. He gets attacked by the traitor legions. They start with a sneak attack on the planet Calth, where they create a massive warp storm (the Ruinstorm), cutting off communication and travel for loyalist legions across the galaxy. The traitors also try to convert some of the surviving loyalist legions – namely Sanguinius and his Blood Angels, and Jaghatai Khan and the White Scars. Horus gets a daemon army, and some of his fellow primarchs ascend to become daemon princes. The loyalists on Ultramar believe that the Emperor is dead and Terra lost, so they declare Ultramar the new seat of the Imperium. After mucking about with alien artifacts, they realise Terra is fine actually. They beat back the traitors, and make haste towards the Emperor.

The third part is the race for Terra and the final siege. Through various hijinks, each of the loyalist legions makes their way towards Terra. Some get there early, and join the defenders on the walls. Some run interference and slow the traitors down. Guilliman and his allies manage to shut down the Ruinstorm, fixing warp travel for the loyalists. The traitor primarchs, now committed to Chaos, are unreliable and distracted – chaotic, even – but they’re slowly corralled in the direction of Terra. Eventually the siege of Terra begins. Horus kills Sanguinius. He fights and cripples the Emperor, who still manages to kill him. Chaos is beaten back, and the empire limps on.