Army of the dead - Definition

In the fictional works of J. R. R. Tolkien, the Army of the Dead were the shades of Men of the White Mountains, who were cursed to remain in Middle-earth by Isildur after they abandoned their oath of aid to him in the War of the Last Alliance. They haunted the caverns beneath the Dwimorberg, and the valley of Harrowdale that lay in its shadow, though they were said to appear in the valley only in times of trouble or death.

In the War of the Ring, Isildur's Heir, Aragorn, called on the Dead Men to fulfil their oath at last. They followed him through the Gondorian lands south of the Mountains, and at the port of Pelargir they drove away the allies of Sauron in fear. For their aid, Aragorn granted them their freedom, and they vanished at last from the world.