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 he or she 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.

Tips
Fellglow Keep can be especially challenging if you are/becoming a vampire because almost every single mage inside the keep uses fire magic and this is one of the weaknesses to the vampire (The higher stage vampire you are, the harder it becomes to sustain your health.). Therefore, you are going to take a lot of damage, and make the quest achieved from Urag gro-Shub even harder than if you're not.

Notable Loot

 * Many various ingredients
 * A slew of Petty Soul Gems.
 * Random enchanted Mage's Robes.
 * 1 Stone of Barenziah, in the keep itself, in the room with the disenchanter and alchemy workbench.
 * 3 Quicksilver Ingots, in the same room
 * A Hypothetical Treachery - Destruction (Skyrim) skillbook, on a table in between the circle of shelves, just before you reach the library.
 * The Doors of Oblivion - Conjuration (Skyrim) skillbook, just after you reach Fellglow Keep from the dungeons.

Enemies

 * Novice Conjurers
 * Novice Ice Mages
 * Novice Fire Mages
 * Novice Storm Mages
 * Novice Necromancers

Quests

 * Hitting the Books

Bugs

 * There are hostile mobs that will sometimes interact through an inpassable pile of rubble. This will sometimes cause your companion to chase after them by running into the rubble till they glitch through, and become stuck in the other zone which is only enterable/exitable through a loading screen door. A way of getting your companion back is to toggle no-colide (type "tcl" in console) fly through the rubble to your companion, and to have them manually walk through the rubble again by selecting the "I need you to do something" chat option.