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Signus Prime
From Warhammer 40k - Lexicanum
As part of Horus' plans to assault Terra during the Horus Heresy, he used his rank of Warmaster to disperse loyalist legions throughout the galaxy to prevent them being able to aid in the defense of Terra.
He commanded the Blood Angels be dispatched to Signus Prime, where a daemon force led by the Greater Daemon of Khorne Ka'Bandha laid in wait to ambush and slay the Blood Angels.
It is indicated, though not confirmed, that this frenzied battle with Chaos forces may have contributed to the Blood Angels developing the dreaded Black Rage.