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The Stone of Erech (also called the Black Stone) was a black stone, somewhere between six and ten feet in diameter*, which Isildur brought from Númenor to Middle-earth in SA 3320. It was set upon the Hill of Erech at the time of Gondor’s founding, and thereafter served as a symbol of its royalty. Upon the Stone, the Men of the White Mountains were required to swear allegiance to the Dúnedain, an oath which the mountain-people later broke. Isildur cursed them to wander the hills, unable to find peace until they had fulfilled their promise. During the War of the Ring (a full age later), the Dead Men of Dunharrow were summoned to the Stone by Aragorn, so that they might fulfill their Oath and have peace at last.
* Some sources say six feet, some say ten; others yet say the stone was twelve feet in diameter
Foster, R. (2001). Stone of Erech. In The complete guide to middle-earth: from the hobbit through the lord of the rings and beyond. New York: Del Rey.