Dunlending Wildmen are a savage race of Men who are possessed with having their vengeance on the people of Rohan who, according to Saruman, drove them into the mountains. In The Two Towers, Saruman uses their anger as a manipulation and convinces them to destroy the land of Rohan during the War of the Ring. He sends them across Rohan, burning and looting the land. 

Soure: lotr.wiki.com