Given to you by Auryen. Reward for having 50 museum displays.
"During the Skyrim Captivity, the master weaponsmith Hilbongard of Skyrim and the enchanter Dorach Gusal created an enchanted warhammer -- feather-light, yet brutal as an atronach's fist. This artifact, christened 'Skull-Crusher' by its makers, was lost when thieves snatched it from a festival display. Legend says its makers gave Skull-Crusher the power to betray unlawful possessors, returning by its own will to its makers." ― Lalatia Varian