Stone of Erech
The Stone of Erech was a great black stone, spherical in shape and roughly six feet in diameter. It was half-buried at the top of the Hill of Erech at the mouth of the Blackroot Vale, far to the west of Minas Tirith. The Stone was a mysterious and eerie place, shunned by the people of the valley, who claimed it had fallen from the sky, and was haunted by restless spirits. In fact, the Stone had its origins in Númenor, and was brought to Erech after the Downfall by Isildur himself.
It was at the Stone of Erech that the King of the Mountain swore allegiance to Isildur's cause in the time of the Last Alliance. When war came, though, he failed to fulfil his oath, and Isildur cursed the King and his people to wander the hills until they made good their promise. After waiting through the long years of the Third Age, their chance came at last when Aragorn Elessar led them out from beneath the Dwimorberg to the Stone of Erech. There at last they fulfilled their ancient bond, and marched to the aid of Gondor.