The Misericordia (High Gothic: "blade of mercy") is a long dagger or short sword that was carried by the Custodians of first the Legio Custodes and after the Horus Heresy, the Adeptus Custodes. The blade was symbolic, intended to represent that the Custodes were beneficiaries of the Magisterium Lex Ultima which placed them above the reach of all Imperial Law save for the commandments of the Emperor of Mankind Himself.
The Misericordia was designed to deliver a single, mortal blow, and despite their symbolic nature, were sometimes used by individual Custodians to carry out a death sentence or offer the Emperor's Peace to a mortally wounded and grievously suffering warrior.
The Misericordia originated on Terra during the Age of Strife, when they became symbols of the authority of the executioner-judges in the service of the tyrannical techno-barbarian warlords of Old Earth who held the ultimate power of life and death over all of their subjects. After the end of the Unification Wars, the Emperor adopted the old practice, but only Imperial agents and judges in the service of the Emperor were now allowed to carry the Misericordia.
More than a lethal sidearm, the Misericordia signifies something greater. As the Emperor led His wars of unification, His Custodians are believed to have co-opted the term for their own use. No longer would the Misericordia be a symbol of tyrannical rule. Instead, it came to represent the right of the bearer to act as the arbiter of the Emperor's judgement, and to put to death those tyrants, lunatics and demagogues who stood against Him.
The Misericordia still shows its wielder to be the Emperor's sanctioned executioner, yet since His fall at the end of the Horus Heresy these blades bear a second, grimmer meaning. They have become weapons of vengeance, to be turned upon those who betrayed the Emperor and left Him a broken shell. Every time a Misericordia is plunged into a Traitor's heart, so it is said, a minuscule measure of revenge is exacted on behalf of the Emperor Himself. Though the Custodians are typically immune to such superstition, there are those amongst their ranks who harbour the hope that if enough Traitor blood is spilt with these blades, it may in some way restore their master.
Another school of thought, the adherents of which are known as the "Miserians," believe that through the wounds inflicted with Misericordia they will slowly bleed the great descendants of Horus, inflicting a death by a thousand cuts upon the Black Legion and its masters. Thus, though Custodians have the right to carry their Misericordia or not as they see fit, it is rare indeed that they go to battle against the Heretic Astartes without these blades at their hips.
- Codex: Adeptus Custodes (8th Edition), pp. 46, 47
- The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 119