Carried by Galmar in Palace of the Kings in Windhelm. If you side with the Stormcloaks, it must be pickpocketed from him. If you side with the Imperials, you get a chance to kill and loot him during the 'Battle for Windhelm'.
"Forged by Galmar when he was still a young lad, he used this warhammer to kill the Frost Troll who mauled his mother to death. As trophy he mounted the monsters head on top of the weapon."