Quote from : The Lord of the Rings: The Return of the King Movie

Gimli: What kind of army would linger in such a place?
Legolas: One that is cursed. Long ago the men of the mountain swore an oath to the last king of Gondor, to come to his aid, to fight. But when the time came, when Gondor's need was dire, they fled, vanishing into the darkness of the mountain. And so Isildur cursed them, never to rest until they had fulfilled their pledge. Who shall call them from the gray twilight, the forgotten people?