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What Started the Horus Heresy?

The Horus Heresy began when Horus Lupercal rebelled against the Emperor of Mankind after falling under the influence of Chaos. His rebellion caused several Space Marine Legions and Primarchs to turn against the Imperium. This betrayal triggered a galaxy-wide civil war.

QUICK FACTS

ItemDetail
EventBeginning of Horus Heresy
Rebel LeaderHorus Lupercal
CauseChaos corruption and rebellion
Initial ConflictIsstvan campaigns
OutcomeGalactic civil war

EXPLANATION

The Horus Heresy began after Horus was corrupted by Chaos during a campaign on Davin. Following his recovery, he began secretly organizing a rebellion against the Emperor. Horus gained the support of several Primarchs and their Space Marine Legions. Early conflicts included the Isstvan III purge and the Drop Site Massacre on Isstvan V. These events revealed the scale of the rebellion and started a massive war across the Imperium.

WHY IT MATTERS

The Horus Heresy is the defining historical event of the Warhammer 40K universe. It reshaped the structure of the Imperium, divided the Space Marine Legions, and permanently weakened humanity’s empire. Its consequences still influence the galaxy ten thousand years later.

RELATED LORE

Who Was Horus Lupercal
Why Did Horus Betray the Emperor
What Was the Siege of Terra
What Happened After the Horus Heresy
Who Fought in the Horus Heresy
Why Did Horus Betray the Emperor
What Happened to Rogal Dorn

FAQ

Where did the Horus Heresy begin?

The rebellion began during the campaigns around Davin and the Isstvan system.

Why did other legions join Horus?

Some legions were influenced by Chaos, while others joined due to loyalty to their Primarchs.

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Baron Von Vault Founder & Gaming Systems Analyst
Baron Von Vault is the founder of the Von Vault Network and creator of D4Dead, BorderlandsHQ, and 40KFAQ. He publishes research-driven analysis of action RPG and looter shooter systems, focusing on progression loops, class mechanics, and endgame design.
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