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. After much conflict, Horus attacks Terra. He cripples the Emperor, who kills him. Chaos is beaten back but not defeated, and the empire limps on.

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 500 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.