The Red Thirst is a genetic flaw deeply embedded into the Blood Angels Gene-Seed. When Sanguinius became aware of it, he felt responsible for the suffering of his sons and desperately sought out a cure while keeping it secret from the rest of the Imperium, including the Emperor himself. A warrior who had succumb too deeply to the Red Thirst or Black Rage was sometimes executed by Sanguinius himself, an act he reluctantly carried out. While isolated instances of Battle-Brothers succumbing to the Red Thirst occurred throughout the Great Crusade, it was not until the Battle of Signus Prime that the entire Legion suffered from it.
The Red Thirst causes warriors to suffer an unusual bloodlust, instilling them with a thirst for the blood of their enemies. Those who succumb to the Red Thirst or Black Rage are either formed into the Death Company or locked away in the Tower of the Lost on Baal, until they change completely, going insane. These traits have also been passed on to their successor chapters, such as the Flesh Tearers, Blood Drinkers, and Angels Sanguine. One successor chapter, the Lamenters, was until recently thought to have managed by unknown means to eliminate the flaws from Sanguinius' gene-seed, but has been stricken by extraordinary bad luck in battle and as such has been nearly annihilated. The Red Thirst is so overwhelming that the Blood Angels forget their noble heritage and completely lose control over themselves. It is not unheard of that Blood Angels squads abandon advantageous positions to engage the enemy in hand-to-hand combat. More often than not, these assaults turn out to be so devastating that unprepared enemy forces are simply swept away.
The origins of the Red Thirst and Black Rage is hotly debated by the Blood Angels and their successors. Some blame the process of Insanguination, others in deep rooted Chaos corruption in Sanguinius as he was transported to Baal. The Mechanicum has observed that Blood Angels have an overactive Omophagea, and this combined with their blood rituals may have caused them to become more strongly affected by past memories than others. Others discuss the impact of Baal Secundus' strange cocktail of chemical toxins and radiation as well as past evidence of mutation among the populace.
Initially it seemed the new wave of Primaris Space Marines were able to more easily resist the Red Thirst, while showing no signs of the Black Rage at all. However the Primaris Marines may not be as resistant to The Flaw as once thought, as evidenced by the Khovan Incident. The hope was ultimately a false one, and Primaris Space Marines have now become members of the Death Company.
- 1: Codex: Blood Angels (4th Edition) - The Flaw
- 2: Fear to Tread (Novel) - Melchior
- 3: Codex: Blood Angels (8th Edition) pg.12-13
- 4: Codex: Blood Angels (9th Edition) pg.11