He was the king of the Men of the White Mountains who during the War of the Last Alliance swore allegiance to Isildur. When they were called upon to aid the king they refused, so breaking their oath.

Isildur cursed them so that they would walk the Paths of the Dead until they had a chance to redeem themselves.

During the War of the Ring, Aragorn called upon The Dead to come to the aid of Gondor, and they did, fulfilling their oath at long last.
Encyclopedia entry originally written by Dark_Ring