The Signus Campaign was a trap baited with duty. Horus despatched the Blood Angels to the distant Signus Cluster on the pretext of liberating its worlds from raiders and xenos, when in truth the entire region had been surrendered to the Ruinous Powers as an abattoir in which to butcher Sanguinius and his Legion far from any hope of aid.
When the Blood Angels fleet arrived, the veil between worlds tore open. The worlds of Signus were awash with daemons, their populations twisted into screaming vessels for the warp, and the sons of Sanguinius found themselves besieged on every side by the seemingly endless legions of Chaos. Worse still, a subtle sorcerous working had sunk its hooks into the very gene-seed of the Legion, waking a red thirst and a black rage that would haunt its warriors until the end of days.
At the black heart of the corruption waited Ka'Bandha, a mighty Bloodthirster of Khorne, who slaughtered hundreds of Astartes in moments to break the primarch's spirit and drown his Legion in despair. Sanguinius met the Greater Daemon in single combat and, though grievously wounded and driven for a time into a killing frenzy, hurled the monster back into the warp and shattered the daemonic host arrayed against him. The Sanguinary Guard fought and died at their primarch's side, and the poisoned ground was carpeted with the slain of both hosts.
The Blood Angels won free of the trap and denied Horus the annihilation he had planned, but they emerged forever changed, their ancient nobility now shadowed by the twin curses of the Flaw. That inheritance of rage and thirst would define the Legion and its successors for all the millennia to come. The campaign is chronicled more fully in the account of the Signus Campaign.