In an attempt to divide those loyalist legions he could not sway to Chaos, Horus dispatched them to the farthest reaches of space, where they unwittingly walked in traps of his and his masters devising. The Blood Angels mission to the Signus Cluster, near to the galactic core in the Ultima Segmentum, was one such venture, where the system of 7 planets and 15 moons had been taken over in bloody slaughter by the daemonic hordes of a Greater Daemon of Slaanesh, Kyriss the Perverse. Thinking that they were arriving to liberate humans from their xeno-overlords, Sanguinius had no reason to suspect that his best friend, the Warmaster Horus, would betray him.
When the Blood Angels fleet arrived in system, they were assaulted by Chaos energies, ravaging their Navigators and Astropaths, killing many outright, as well sending mad panic through the Legion's ship serfs and auxiliaries, who until then had not ever felt the effects of Chaos. Most Space Marines were immune to its effects due to their training and iron-will, leaving those few not driven mad to restrain the crazed crew members and erect psychic baffles. The daemon conqueror of Signus, Kyriss, had time however to beam an image of itself to the Legion's Primarch, challenging him to take back Slaanesh's newest conquest. At this Sanguinius gathered his senior officers and organised a counterattack against a foe they had no experience fighting, however, sure of their victory, Sanguinius declared This foul creature will know what it is to face my wrath. I shall strike him down, I shall be an Angel of Vengeance!
With Signus Prime covered from pole to pole with the result of daemonic invasion, with millions lying dead in the streets or held in prison camps to await daemonic whims, with cities razed, oceans boiled away and plains burn to cinders, the Blood Angels Legion struck in a furious counterattack, falling from the skies to engage crazed cultists and packs of hideous daemons upon the Plains of the Damned. Sanguinius swore to personally cast Kyriss down and vanquish him, so hence made for the Cathedral of the Mark, the daemon's lair. However outside its entrance, Sanguinius was faced by the single greatest foe who would have the greatest effect on his Legion and its descendants down the millenia, Ka'Bandha, Greater Daemon and Bloodthirster of Khorne.
During the conflict Sanguinius was nearly killed by the mighty Bloodthirster Lord Ka'Bandha. The Primarch's legs were crushed and broken, but instead of killing the defenseless Primarch, the daemon said "Your legs will heal but this wound will always fester." Then, as Sanguinius was powerless to help them, hundreds of the Primarch's Astartes were slaughtered by the daemon in a single rampage. The psychic backlash of so many Loyalist deaths at once caused Sanguinius to black out. While their Primarch was unconscious, the remaining Blood Angels felt a raging madness take over them as they slew the remaining Chaos cultists and daemonic legions without mercy. This, as much as Sanguinius' death, is the cause of the Black Rage. The daemonic legions of Khorne, normally nigh-invincible, were banished, and Ka'Bandha quit the field. When Sanguinius awoke, the surviving Space Marines were sullen and despairing, the berserker rage they experienced having left a brooding shadow on their souls. Shortly after, the Blood Angels were recalled to Terra to aid the defence of the Imperial Palace from the Traitor Legions, leaving behind grim memories of Signus Prime.
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