Lamistard's tunnels are a series of artificial tunnels located underneath Zemouregal's fort. It is visited during The Curse of Arrav quest. It was created by the Mahjarrat Lamistard, in an attempt to hide underneath The Ritual site, much like how Jhallan hid inside the natural caves under the ritual site in the Fremennik Province. Eventually, Lamistard accidentally burrowed in the cellar of Zemouregal's fort, where Zemouregal captured him and kept him prisoner until he was sacrificed at the 16th Ritual.
The only entrance to the tunnels is on the eastern side of Trollweiss mountain. The cave has experienced some recent cave-ins, and will require you to mine through some of the rubble in order to navigate the tunnels (64 mining required).