The Wildmen of Dunland are dirty hillmen from eastern Rohan. 500 years before the War of the Ring, Gondor gave the lands of the Mark to Eorl the Young and made an alliance with him. The Men of Dunland were therefore driven out by the Rohirrim and have always sought revenge. In The Two Towers at the Battle of Helm's Deep, Gamling reveals that he has knowledge of the language of the Men of Dunland, calling them fierce warriors whose hatred of the Rohirrim runs deep. It is for this reason that they were easily persuaded by Saruman to fight for him.
In Edain Mod, the Dunland sub-faction of Isengard can be found in the Clan Steading expansion available at Settlements.