The Dead Men of Dunharrow:

Ghosts, fated never to rest until they fulfill an oath they broke to Isildur. They fulfilled that oath in the days of Aragorn, during the War of the Ring in 3019, after which they never disturbed Middle-Earth again. Freed from the curse laid upon them by Isildur, they probably left the Circles of the World as is the fate for all of the Atani.

The oath sworn by those who later became the Dead men of Dunharrow was to aid Isildur in his fight against Sauron. When the time came for them to fight for him, he found that they had allied themselves with Sauron. However, his curse so frightened them that they remained at home, and fought on neither side of the war.