Paths of the Dead
The Paths of the Dead were a dark and dreadful road that passed beneath the mountains of the White Mountains from the vale of Harrowdale in Rohan to the lands of Gondor beyond, haunted by the restless spirits of the Dead. The passage of these paths by Aragorn during the War of the Ring, and his summoning of the ghostly army that dwelt within them, proved a decisive stroke in the deliverance of Gondor.
The Curse of the Dead
The shades that haunted the Paths of the Dead were the spirits of Men of the White Mountains, the Dead Men of Dunharrow, who in the Second Age had sworn allegiance to Isildur but had broken their oath, refusing to aid him in the war against Sauron because they had once worshipped the Dark Lord. For this betrayal Isildur laid a curse upon them: that they should have no rest until they had fulfilled their oath. Unable to die in peace and unable to fulfill a vow long impossible, they lingered as restless ghosts beneath the mountains, and the road they haunted became a place of terror that no living man dared enter.
Aragorn's Passage
As heir of Isildur and the rightful king of Gondor, Aragorn alone had the authority to call the oathbreakers to their long-deferred duty. Pressed for time, with the army of Mordor descending upon Minas Tirith, he chose the swiftest and most terrible road. With his companions Legolas and Gimli and a band of the Dúnedain, Aragorn entered the haunted gate beneath the mountain at Dunharrow and passed through the dark, where the dead followed silent and unseen.
Coming out beyond the mountains, Aragorn unfurled his banner and summoned the Dead to fulfill their oath: to follow him to battle and so be released from the curse. The shadow-host obeyed.
Deliverance of Gondor
The Army of the Dead followed Aragorn to the river-port of Pelargir, where the Corsairs of Umbar lay with their black fleet, sailing to reinforce Mordor. The terror of the Dead swept the Corsairs from their ships without a blow. With their oath thus fulfilled, Aragorn released the Dead from their curse, and they passed at last to their rest. Manning the captured ships with the freed men of southern Gondor, Aragorn sailed up the Anduin and arrived at the Battle of the Pelennor Fields at the crucial hour, turning the tide of the battle.
Significance
The Paths of the Dead are bound up with the redemption of an ancient oath and the proving of Aragorn's kingship, for only the true heir of Isildur could command the oathbreakers. Aragorn's daring passage and his summoning of the Dead brought timely deliverance to Gondor and stand among the boldest deeds of the War of the Ring.