Before leaving to inspect the orb you found in Saarthal, the Arch-mage will tell you to speak with Urag gro-Shub, who will probably be in the Arcaneum with his books. Urag will ask you to retrieve some books that were stolen by a mage called Orthorn who ran off to Fellglow Keep with a group of renegade mages. He’ll mark Fellglow Keep on your map for you.
The Keep is in a wilderness area located downriver from the city of Whiterun. When you arrive, battle two leveled mages and a Flame Atronach, then head through the entrance into the dungeon. This place is populated by frostbite spiders as well as various mages who will attack with elemental magic and conjured minions. They are unarmored and therefore vulnerable to combat weapons. If wounded badly they will flee and you’ll have to chase them. Each wears magic robes, useful for selling or enchanting. Many are also carrying scrolls, potions, and alchemy ingredients.
You will soon encounter a torture chamber where the renegade mages are keeping some vampires prisoner. These vampires are not hostile towards you, and will attack the mages if you set them free. Once you push past the torture chamber you will see Orthorn imprisoned in another cage. He will beg you to set him free. If you do so, he will offer to come with you. It can be difficult to hide Orthorn from his jailer, who will release two wolves when he notices the mage is free, so it is much easier if you kill the jailer before releasing Orthorn.
You will then pass into a training room containing three additional mages, and then into a chamber and up to a sort of library full of Ruined Books and yet more mages. The library contains a skill book on a table; make sure you read or take it before moving on. You will also find a room to your right which contains an enchanting table, an alchemy lab and some smithing equipment.
After the library, dispose of an Atronach, then enter a chamber where you’ll be greeted by The Caller, the leader of the renegade mages. She can be a difficult opponent, especially for low level characters. She possesses one or two summoned atronachs (the type and number depending on your level) and wields a range of destruction spells including Sparks, Firebolt and Ice Spike.
When you first come across her, she will initiate dialogue with you, and when you tell her what you have come here for, she will offer to allow you to take the books but then asks that you leave Orthorn with her. You can refuse outright and commence the fight, or, alternatively, you can pretend to accept this, then as you move past her and go to take the books, deal her a sneak attack from behind, badly wounding her or possibly killing her with that one hit.
There is also a [Persuade] dialogue option which may cause her to allow you to take the books. Using this approach causes Orthorn to turn around and leave the room, never to be seen again. If engaged in conversation he will confusingly reply with “How could you just leave me here?” as if you had made the choice to swap him for the books.
If you have an invisibility spell or potion, you can use it immediately following the conversation, before she conjures up any of her minions, then sneak up behind her and try to eliminate her with one critical blow (this may depend on your sneak skill).
If she does not allow you to leave with the books, take the key to open the exit door off of her corpse after you’ve killed her.
If you have not yet released Orthorn and come back to where he is being held to do so before you leave this place, he will thank you and then leave the dungeon. He will actually attempt to run all the way across Skyrim back to Winterhold, but will usually die from random encounters along the way.
Upon returning the books to Urag gro-Shub, he will reward you with several skill books that will raise your Conjuration, Enchanting, Alteration, Restoration, Destruction and Illusion skill levels.