Deepholt was a legendary underground city that was mentioned by Meachreasim Illistim in his "History of Elanthia. Virtually nothing is known about it, aside from being founded by Osriar Melenthrope. Many believed that Rone Wehnimer was seeking it when he was last seen traveling southeast with a pack-train of supplies from Wehnimer's Landing.
Behind The Scenes
The original history document of Elanthia was filled with hooks for further expansion. Deepholt was apparently never expanded upon, and nothing has been established about Osriar Melenthrope. It might be an homage to "Dimholt" from Lord of the Rings, a dark forest that led to lost cities under the mountains, where Aragorn went to acquire his undead army to fight Sauron.