The Blood Angels and all the Space Marines of their Successor Chapters generally have pale skin, moderately elongated eye teeth and a strong urge to drink the blood of their enemies which can grow stronger over the solar decades of their service, eventually causing degeneration into uncontrollable madness.
Deep within the psyche of every Blood Angel and every bearer of their Primarch Sanguinius' genetic legacy is a destructive yearning, a battle fury and blood-hunger that must be held in abeyance in every waking moment.
Few battle-brothers can hold this Red Thirst in check unceasingly -- it is far from unknown for Blood Angels to temporarily succumb to its lure at the height of battle.
The Red Thirst is the Blood Angels' darkest secret and greatest curse, but it is also their greatest salvation, for it brings with it a humility and understanding of their own failings which make them truly the most noble of the Space Marines.
The fate of those unfortunates overtaken completely by the Red Thirst is known only to the Chapter itself. There are tales of a secret chamber atop the Tower of Amareo on Baal in their fortress-monastery complex of the Arx Angelicum, and of howling cries that demand the blood of the living, but none are willing to say for certain what secrets lie hidden in that haunted, desolate place.
There have been incidents when the Blood Angels have been stationed on distant worlds where members of the local population have gone missing only to turn up later drained of blood. It is possible that this is the work of cultists seeking to discredit the Chapter.
It may even be that some of the more superstitious local citizens have taken to offering up sacrifices to their godlike visitors. It may also be possible that these folk have been killed by Blood Angels overcome by the Red Thirst.
All Space Marines are a dim reflection of the greatness of their individual primarchs, for they are the inheritors of their gene-father's genetic legacy. Unknown to the Sons of Baal, there was a hidden flaw in the genetic matrix of the Blood Angels Legion's gene-seed.
Within Sanguinius' own genotype there was a trait that lay buried and waited to be awakened. During the Great Crusade, this strange affliction began to manifest, affecting some of the Blood Angels Legionaries over the course of several solar decades.
The story was always the same -- a warrior of the IX Legion in the throes of battle eventually succumbed to a rage that continued to build and build until his reason was lost. When a Blood Angels battle-brother succumbed to this affliction, his Humanity would be stripped away until only his feral core remained and all the blood-crazed Astartes wanted to do was kill and kill, satiating himself with blood and more blood.
At the end, at the very worst of it, he lost every last piece of himself, until death was a kindness. Fortunately, only a handful of Astartes had been afflicted over the course of two Terran centuries of continuous conflict. Most would perish in battle without anyone taking note of their growing insanity.
But if their degeneration became noticeable, then Sanguinius himself, or the Sanguinary Guard's commander Azkaellon, would usually be the one to end the life of the afflicted battle-brother. Yet, in the closing days of the Great Crusade, the outbreaks of this affliction began to occur more often. Sanguinius feared that, in time, this affliction would grow to encompass every member of the IX Legion.
Sanguinius had been aware of the Flaw in his genome for several years, keeping the truth from the Emperor and his fellow primarchs. He could not speak of this to any of the others, for to do so would diminish his Legion in the eyes of his brothers and the Imperium.
Some of his brother primarchs would see it as a weakness and seek to turn this truth against him. Sanguinius was afraid to confide in his father the Emperor for he could not take the risk of being responsible for the erasure of the Blood Angels from Imperial history like the II and XI Legions.
Sanguinius continued to search for a solution but continued to fail in this endeavour. Some of the Angel's sons had learned a measure of the truth, but only Azkaellon, First Captain Raldoron, the IX Legion's Master Apothecary on Baal and a few others were fully aware of the extent of this affliction.
They were united with Sanguinius in finding a way of repairing this flaw, but ultimately this quest proved futile before the tragedy of the Horus Heresy swept away all other concerns.
As a result, from the day of their Sanguination to their death on the battlefield, the Blood Angels and their successors fight not only with countless foes but with a burning urge inside them. This is the Red Thirst, the legacy of their Primarch Sanguinius, that plagues many Blood Angels with visions of death.
In battle Blood Angels can harness these visions to evoke a ferocity that gives the Blood Angels Chapter its reputation for unparalleled brutal assaults. Sometimes, however, 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 for Blood Angels squads to 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 Red Thirst is one of two genetic flaws to be found in the gene-seed of the Blood Angels and their Successor Chapters. Those who succumb to the Red Thirst or the other known flaw, the Black Rage, are either formed into the Death Company or locked away in the Tower of the Lost on Baal, until they succumb completely to a roiling, uncontrollable insanity.
These traits have also been passed on to the Blood Angels' many Successor Chapters. One such Successor, the Lamenters, managed by unknown means to eliminate the flaws from Sanguinius' gene-seed, but has been stricken by extraordinary bad luck in battle and were nearly annihilated before receiving Primaris Space Marine reinforcements during the Indomitus Crusade.
- Codex Adeptus Astartes - Blood Angels (8th Edition), pp. 12-13
- Codex: Blood Angels (5th Edition), pp. 12-13
- Codex: Blood Angels (4th Edition), "The Flaw," "The Red Thirst"
- Fear to Tread (Novel) by James Swallow
- Bloodquest I by Gordon Rennie and illustrated by Colin MacNeil
- Bloodquest II: Into the Eye of Terror by Gordon Rennie and illustrated by Colin MacNeil
- Bloodquest III: The Daemon's Mark by Gordon Rennie and illustrated by Colin MacNeil
- Bloodquest: Prisoners of the Eye of Terror (Audio Book)
- Blood Angels Novel Series:
- Deus Encarmine (Novel) by James Swallow
- Deus Sanguinius (Novel) by James Swallow
- Red Fury (Novel) by James Swallow
- Black Tide (Novel) by James Swallow
- Bloodline (Short Story) by James Swallow