It was later discovered that the Dunlendings had secretly been moving back into the northwestern uplands of Rohan throughout the reigns of Déor's predecessors. By Déor's own time, a powerful Dunlendish force had been established, and the King was forced to ride northward from Edoras, ultimately to defeat his enemies. The victory was a bitter one, though, for he discovered that the Dunlendings had also captured the Ring of Isengard, which Déor had no possible means of recapturing. So Déor's reign saw the beginning of a danger from the north that would reach a crisis nearly fifty years later, when the Dunlendings were to come close to destroying Rohan alogether.
Déor's turbulent reign lasted nineteen years. He was succeeded as King by his son, Gram.