Fellglow Keep is a location in The Elder Scrolls V: Skyrim. Led by Orthorn, mages that left The College of Winterhold fled to this location to practice forbidden experiments. Urag gro-Shub sends the Dragonborn here in search of three books to help research the strange orb found at Saarthal. The mages keep Vampire test subjects caged in the halls below, using them to perform cruel surgical experiments, as well as using them as the targets for practicing Destruction spells. The vampires can be freed, but if Orhtorn was released, he attempts to kill them immediately upon sight.
Searching for the books, the Dragonborn discovers that the other mages turned on Orthorn, caging him. If the player frees him, he fights alongside him or her as a Companion. He uses the spells Firebolt, Conjure Familiar, and Conjure Flame Atronach. He reveals that a mage known simply as The Caller imprisoned him after stealing the books.