Dwimorberg was part of the White Mountains, which was situated at the head of the valley of Harrowdale. Dunharrow stood above a cliff on the valley's eastern side, parallel to the Dwimorberg Mountain itself. Beyond Dunharrow was a forest of pine wood and fir, known as the Dimholt. It was the location of the Dark Door.
The Dimholt itself contained a small glen, which held a relic of Númenor, a standing stone, that lay in front of a doorway that led to the Paths of the Dead. The paths led under the Dwimorberg mountain and were in the Third Age haunted by the spirits of the men of the mountains. The men of the mountain swore an oath to Gondor and Isildur, the King of Gondor in those days. But they were descended from an old race that had lived in Eriador in the Dark Years, before Númenor was. And their fathers had worshipped Sauron in the Dark Years, and still had some love for him, and so when called upon to fulfill their oath to Gondor, they would not -- and so Isildur cursed them, never to rest until they had fulfilled their oaths.
Dwimorberg was a Rohirric word that meant 'haunted-mountain'.