The Paths of the Dead was a haunted pass through the White Mountains (Ered Nimrais).
The Paths of the Dead started at the Dark Door at the end of the long valley of Harrowdale, beyond the Firienfeld and the forest of Dimholt, wedged in between the mountains Irensaga (Iron Saw), Starkhorn, and the Dwimorberg.
After the Dark Door, the Paths went under the Dwimorberg, past another door where Aragorn and company found the remains of Baldor, son of the second King of Rohan, Brego. The Paths then continued past forgotten cities, emerging at the southern end of the White Mountains in Morthond vale, near the Stone of Erech.
In the Second Age, a people related to the Dunlendings had lived in the White Mountains, and they had for a time served the Dark Lord Sauron. They later swore allegiance to Isildur of Gondor and Arnor, but betrayed him during the Last Alliance of Elves and Men and refused to fight on either side. For this reason, Isildur cursed them, proclaiming that, if the Alliance triumphed, they would linger in the mountains until one of his heirs called them again to fight against Sauron.
Isildur's curse succeeded: their people dwindled and eventually died out, until only their ghosts remained at the end of the Third Age. When Aragorn and company took the Paths of the Dead and, deep within the mountains, summoned them to the Stone of Erech, their shades followed him. At the Stone at midnight, Aragorn unfurled his banner, declared himself the heir of Isildur, and commanded the dead to aid him; they obeyed, and defeated the armies of the Corsairs of Umbar who were attacking the landings in southern Gondor. (Supposedly, the only weapon that they required was fear, driving their enemies to cast themselves madly into the sea.) When those foes were defeated and Aragorn could claim the black ships of the Corsairs for his own, he declared that the curse was lifted, and the dead departed forever. When the dead had gone, the men of southern Gondor joined Aragorn in the ships and sailed to the aid of Minas Tirith.
In the theatrical release of the Peter Jackson movie adaptation of The Lord of the Rings: The Return of the King the battle with the Corsairs occurred off-screen and the Army of the Dead accompanied Aragorn to Minas Tirith to defeat Sauron's orcs, after which Aragorn declared their curse lifted. Note: The extended edition of the movie shows the journey through the Paths of the Dead, then a small portion of the battle with Aragorn confronting the Corsairs and warning them not to go any further. When they defy him, the ghosts proceed to board the ships, visibly striking fear into the pirates' hearts.
The Paths were depicted with an unnatural mist, mass crypts and piles of skulls. As Aragorn and company travelled through, Legolas could sense the dead closing in behind them. When they arrived at the unnamed Stone of Erech, the Army of the Dead surrounded them until Aragorn threatened them with a reforged Narsil, holding them to their oaths. After an apparent refusal, the massive crypt behind them exploded, as if to bury them in skulls. They escaped and the army appeared by them and swore to fight.
The fell riders of the "Furious Host" of the restless dead, the "Wild Hunt" that may burst upon the unwary traveller in lonely places, is the theme from northern European folklore that Tolkien evoked, vividly realized and built upon. Horses, knights, hounds are among the restless spirits in the inhuman train. Hans Sachs' poem, "Das wutend heer der kleinen dieb" (1539) describes the furious host in gruesome detail, accompanied by ravens who were plucking out the eyes of the roving dead, till at last "there came one behind, who had been hanged the same day, had still his eyes and saw me." It is a measure of Aragorn's heroic nature that he dares summon the dead to account in this manner, and a superhuman sign that the rightful king has indeed returned.