The Signus Campaign was a deadly trap sprung upon the Blood Angels during the Horus Heresy, engineered to destroy one of the Emperor's most beloved Legions far from the decisive battles. Lured to the Signus Cluster by false pretences, the sons of Sanguinius found the region overrun not by mortal foes but by the daemonic hosts of Chaos.
The Blood Angels fought their way through a nightmare of warp-spawned horrors, their discipline tested against enemies that defied reason and reality alike. The strain of the campaign awoke something terrible within them, an ancestral flaw in their gene-seed that manifested as a murderous, blood-hungering frenzy. Warriors succumbed to a red thirst and a black rage that would haunt the Chapter and its successors for all time thereafter.
At the campaign's climax, their primarch Sanguinius confronted a mighty Bloodthirster of Khorne, a duel that pushed the angelic primarch to the very edge of ruin. Though the Blood Angels ultimately prevailed and broke free of the trap, the victory came at a grievous cost in lives and in the scars left upon their souls. The Signus Campaign had failed to annihilate them, but it had wounded them in a manner no battle could heal, planting the seeds of a curse that defines them to this day.