Death Company
The Death Company is a unit used by Blood Angels and their successor Chapters, composed of those Marines who have succumbed to the Red Thirst and the Black Rage.
During the Great Crusade, the Blood Angels discovered that their gene-seed had a flaw that caused some Marines to develop a literal thirst for blood and devolve into raving maniacs, to the point that the Emperor's Peace (i.e. euthanasia) had to be administered. The so-called "Red Thirst" became a dark secret of the Legion, covered up from all outsiders, including the Emperor himself, as Sanguinius feared that his Legion would be exterminated if the news leaked. The numbers of those afflicted remained low, but then the Horus Heresy hit. The psychic backlash from Sanguinius's death caused (and still causes) the "Black Rage", where Marines relive the events of his death at the hands of Horus so vividly that they believed they were Sanguinius. Knowing that the Black Rage and Red Thirst were occurring too frequently to be ignored, but wanting to give their battle-brothers an honourable death, the Blood Angels and their successors formed what they called "Death Companies", Marines overseen by the Company Chaplains to serve as front line shock assault troops. The Death Company will proceed to rip and tear anything in their path until death claims them. Those few who survive are taken back to the fortress-monastery and filled with enough drugs to keep them from attacking everything else until the next time they are called into battle.