Yannisara the arch-sorcerer originally created the magical Stonebridge Warhammer as a reward for for the Redweed Dwarves, after the latter saved Yannisara's life during a great battle.  It was given to one of Gillibran Ironhelm III's ancestors; Gillibran inherited the Warhammer from his father.
If thrown at an enemy, the Warhammer will return to its wielder's hand after hitting its target. The Warhammer also serves as a standard for the Dwarves of Stonebridge during battle, who rally around the Warhammer's bearer whenever Stonebridge is attacked.  The Warhammer also has the power to destroy evil magical artifacts.