Hollow Mountain

The Hollow Mountain is a vault located under Mt. Telkin. Its entrance is guarded by the Den's Fort village. The former inhabitants of the vault were the, today only roam its halls.

visits it during one of the following quests:

He finds the inside it and learns that the  worked together with the  inside the vault.

''The Hollow Mountain is a large, open space, dug out by a now-extinct mole-like species called the Mohlen. They started excavating the mountain because of a very rich and important material, which they deemed Mohlenite. It has conducting properties, making it useful for moving objects and powering their machinery inside the mountain.''