Hill of Erech
The Hill of Erech was a hill lying on the southern fringes of the White Mountains, near the source of the River Morthond. When Isildur came to Middle-earth, he brought with him a great Black Stone, which he placed on the summit of the Hill of Erech (and which was afterwards known as the Stone of Erech).
It was on the hill that the King of the Mountain swore his aid to Isildur, but when he failed in his promise, he and his people were cursed to wander the mountains as the Dead. So the hill and the stone gained a sinister reputation among the people of the Blackroot Vale where it stood, as a place where the whispering Dead would meet. At the time of the War of the Ring, Aragorn led the King of the Dead and his people to the Hill one last time, and there they granted their aid to Isildur's Heir, and finally fulfilled their ancient oath.