The Oath-Broken are a special formation of the Black Legion comprised of those Chaos Space Marines who have failed in the service of the Legion's master, Abaddon the Despoiler. Those that dare betray the Warmaster of Chaos seldom live long enough to regret their mistake, and those who do quickly come to regret their folly bitterly as their bodies and souls are subjected to the most abject of agonies. The Oath-Broken are no true traitors to Abaddon, but merely those who failed in their assigned tasks, or got badly mauled on the battlefield and crippled by horrendous wounds. Failures and cripples invariably lose the fickle attention of their mortal and immortal masters alike, and without the blessings of the Powers of the Warp to mend their limbs or knit their broken bodies together with mutations, the Oath-Broken are forced to fashion their own crude replacements: blades sutured to stumps, xenos appendages crudely grafted into empty sockets and ragged armour patched with whatever material the Eye of Terror deems to provide. And yet, woe to the fool who underestimates these dregs of the Black Legion, for an Oath-Broken's desire to again bask in the glories that only Chaos can provide is only equal to his hatred of any who still do.
- Black Legion: A Codex: Chaos Space Marines Supplement (6th Edition) (Digital Edition), pp. 55-63