The Red Thirst is a genetic curse afflicting the Blood Angels and their successor Chapters of the Space Marines, a flaw woven into their gene-seed by the death of their Primarch Sanguinius. In the heat of battle, a warrior so cursed may be overtaken by a raging, unquenchable hunger for blood and slaughter, his mind flooded with the phantom sensations of his father's final duel against the Arch-Traitor.
When the Thirst takes hold, an Astartes fights with terrible fury, hurling himself upon the foe with fangs bared and blade raised. Some masters harness this rage, channelling it into the disciplined ferocity of the assault. Yet the Thirst is a precursor to a darker doom, for those who surrender to it entirely may fall into the Black Rage and be lost forever.
The Chaplains of the afflicted Chapters watch their brothers ceaselessly for signs of the curse, for the line between righteous fury and damnation is perilously thin, and the blood of Sanguinius runs hot in every son he left behind.