Wulf
Wulf was the son of the Dunlending Chieftain Freca. In Third Age 2754, Freca attempted to arrange a marriage between Wulf and the daughter of Helm Hammerhand. When Helm refused, Freca reviled and raged against him. Angered, Helm struck Freca so hard it killed him.
Four years later, Rohan was invaded from the East. Noticing the Rohirrim were distracted, Wulf seized his chance to take the throne. He drove Helm into the Hornburg, then captured Edoras and sat on the throne in Meduseld.
During the following Long Winter, Helm Hammerhand and both his sons froze to death after a long siege in Helm's Deep. Helm's nephew Fréaláf snuck into Meduseld with a small party and killed Wulf, to the great rage of the Dunlendings.
This was the start of the animosity between the Rohirrim and the Dunlendings that would last for centuries.