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 in 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 Astorath the Grim lets them re-roll failed to hit and to wound rolls, they are among the deadliest assault troops in the game.
Something to be Said about the Red Thirst
Although sometimes misconstrued as the "lesser" of the two curses in the manner of "out of the two times I took chainsword to the rectum, this one didn't hurt as bad" many make the erroneous assumption that the Red Thirst is anything but permanent. As a matter of fact it was the thirst that pulled Chapter Master Caedis (pictured above) over the edge. And most if not all Blood Angels who go over the edge due to the Red Thirst are viewed with measurably more pity than those who did because of the rage. The reason for this being because the Rage is inevitable--Chaplains and the Sanguinary Priests can coax a Battle Brother from back over the "point of no return" quite a few times before the Rage fully takes them. But when a Brother-Marine is lost to the Thirst it is a reflection of his own self-control (And those that know Mephiston's story know that somehow he was able to rein back control from the Thirst just by being buried under rubble for a week, forced to fight his demons alone. Thus, he is viewed as the most badass Librarian ever, capable of being so mad that he's practically serene). This has the unhealthy side-effect of producing Heroic Martyrs in the Chapter who would prefer to take out as many enemies of Man with them before they turn into a slavering, rabid shell of their former self. Those who do not die and do not meet death at the end of the High Chaplain's Axe, are locked within the dark Tower of Amareo to howl in anger and torment until their mortal bodies expire.
BUT
And it's a big one. If the new Index Astartes regarding the Death Company is to be believed (which it is), the brothers trapped in the tower tend to accumulate over the years along with the brothers who are in stasis from the Death Company. And in the Chapter's most dire moments of need, they will suit up EVERY. SINGLE. ONE. of these rabid, rage-fueled motherfuckers and bring true meaning to the the term Death COMPANY. Where the Blood Angels themselves (whose gene-seed is still relatively stable, compared to the other First Founding Legions) can field more than an entire battle company after saving up over the years, Successor Chapters like the Angels Sanguine or Angels Encarmine can field up to three hundred of these blood crazed maniacs, and the Flesh Tearers likely have twice that.