Army of the Dead

The Dead Men of Dunharrow (also referred as the Shadow Host, the Grey Host, the oathbreakers, or simply the Dead) are fictional characters in J. R. R. Tolkien's Middle-earth legendarium. They appear in The Lord of the Rings as the ghosts of Men of the White Mountains (Ered Nimrais), who were cursed to remain in Middle-earth by Isildur after they abandoned their oath to aid him in the War of the Last Alliance. They were formerly known as the Men of the Mountains, and they were related to the Dunlendings.

At the end of the Second Age, their king, known as the King of the Mountains, pledged allegiance to Isildur at the Stone of Erech. However, these Men later refused to aid Isildur in his war against Sauron; they came to the aid of neither side, but instead hid in the mountains. They had previously worshipped the Dark Lord during the Dark Years. As punishment, Isildur cursed them, saying that they would not have peace or rest till they fulfilled their oath upon his command or that of his heirs.

Their spirits haunted the caverns beneath the Dwimorberg, the Haunted Mountain, and the valley of Harrowdale that lay in its shadow. They were said to appear in Harrowdale only in times of trouble or death, and they haunted the Stone of Erech on the other side of the mountains as well.

Malbeth the Seer prophesied that a day would come when need and haste would drive one of Isildur's heirs to take the Paths of the Dead and that the Dead would answer to his call. Thousands of years later, in the final years of the Third Age, that prophecy was fulfilled. In the War of the Ring, Aragorn, Isildur's heir and direct descendant, called upon the Dead, summoning them to the stone of Erech, and commanded them to fulfil their oath and be free. On this occasion, he first displays the royal banner of Gondor, made by Arwen and delivered to him by Halbarad and the Grey Company.

...
Wikipedia