The War under the Mountains was a war between the Dark Elves and the Dwarves that lasted from 586 to 702 YSD (116 years, making it the longest in Ashan's Historical Age). When the Dark Elves were banished from Irollan, they went underground and eventually settled on the borders of Grimheim. The Dwarves were not happy about this, and as the two races competed for the same resources, they started skirmishes against one another that soon turned to all-out war.
The rune priest Hangvul attempted to convince the Dark Elves to leave by destroying their sanctuaries to Malassa. As revenge, the warlock Agbeth captured and tortured him, taking his left eye and left arm. Hangvul later crafter metallic replacements for his lost body parts.
Massive armies clashed in the dark, neither side showing mercy to their opponent. Eventually, King Hathor led his forces in a last assault that crushed the Dark Elves and drove them away. The Dark Elves fled on, eventually settling in Ygg-Chall.
During the war, exiled demonists from the Seven Cities made contact with the Dark Elves and offered them assistance, if they would agree to worship Urgash. Most of the Dark Elves refused, but Raelag, leader of the Soulscar clan, signed a pact with the cultists and began to worship Urgash in secret.