- "Ulf is demanding blood money, which Shagga and Gunthor refuse to pay."
- ―Tyrion Lannister
Ulf is the leader of the Moon Brothers, one of the hill tribes of the Mountains of the Moon recruited by Tyrion Lannister to fight for him in the initial stages of the War of the Five Kings in exchange for gold, weaponry, and armor the hill tribes could employ against House Arryn.
Tyrion reports to his father and his uncle, that a Moon Brother stabbed a Stone Crow over a sausage; in retaliation, three Stone Crows seized the Moon brother and opened his throat; Ulf demands blood money, but Shagga and Gunthor refuse to pay.
In the books
In the A Song of Ice and Fire novels, Ulf son of Umar, is also the leader of the Moon Brothers, who accompany Tyrion to his father's encampment on the promise of weapons and armors.
Ulf is killed at the Battle of the Green Fork. Tyrion finds him laying in a pool of congealing blood, his arm gone at the elbow, a dozen of his Moon Brothers sprawl around him.
Ulf is succeeded by Crawn son of Calor as the leader of the Moon Brothers.