Misericorde is obtained by purchasing it at the Weapons Shop for 34,200 Gold. However, it needs to be obtained first before it'll appear in the Weapons Shop's listing. There are two ways to do this:
A misericorde was a narrow dagger used in medieval times. Its name is an old French word from the Latin misericordia, "mercy", and was named mercygiver in England. This weapon was used to dispatch knights who had received mortal wounds not treatable on the battlefield, which were not always quickly fatal.