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 and are now batshit insane motherfuckers with a short lifespan.
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.
On a side-note, not all Death Company are completely off the reservation when they take up the red and black. Just mostly crazy. Usually the Chaplains of the chapter will find those who can't contain the bloodlust anymore and give them two options: The Red and Black where they use the last of their sanity following the Chaplain into a glorious suicide charge, or the Emperor's Peace where they...don't. Obviously most choose the former as no Son of Sanguinius would ever give up the opportunity for one last chance to tear a foe limb from limb.
On the tabletop, Death Company are, death. They have Furious Charge and Feel No Pain, and the Rage rule that held people back from using them n the 5th edition is now turned into a strength giving them an extra attack while charging. Combined with their wargear options, along with attaching a Reclusiarch or Chaplain lets them re-roll failed to hit and to wound rolls, they are among the deadliest assault troops in the game.