You will have to fight some more Mages in the first room of the Keep (screen above). I'd suggest starting off by attacking the Warlock, as he's capable of summoning magical underlings. After the Necromancer appears, focus your attention on him, as otherwise he will resurrect his fallen companions. Afterwards examine the area to find The Doors to Oblivion, boosting your Conjuration.
Continue exploring the Keep, heading east. After reaching the room with a set of stairs, it would be good to surprise the Mages in the area (screen above). Afterwards you can take care of securing the ground floor, coming across an enemy Necromancer while you're at it, or take the stairs onto the upper balconies.
You should take a look around the area here as well. The western room contains a number of precious treasures, as well as Alchemy and Smithing stations. The more important thing is however to choose the eastern corridor (screen above) leading to the library. Be ready to deal with some Mages, who you can fight either inside the library or in the narrow corridors, depending on which you prefer.
Inside the new room you will find the A Hypothetical Treachery skillbook, boosting your Destruction. Choose the southern corridor. You can of course examine the small room that you come across on your way, though it's purely optional. The more important part is checking out the second side room. Attack the Storm Mage (screen above) by surprise and examine hid corpse to find the Fellglow Keep Key.
Open the door and quickly deal with the Atronach. While you're at it, I'd also suggest examining the eastern room. There you will find the Shire of Julianos (screen above), thanks to which you can heal all diseases and additionally gain a blessing.
Use the stairs to reach the upper level. After getting there, look around the shelves for precious potions and scrolls. I'd suggest saving your game, as you're about to meet the main character of this quest. Use the door to the Fellglow Keep Ritual Chamber (screen above).
After reaching the destination, a conversation with The Caller, who's rather unhappy with you killing all of her Mages, will automatically start. You should of course inform her that you're interested in the books stolen by Orthorn. In the end you should consider trying to make a trade (screen above). The Caller will want Orthorn to remain in the Keep as a test puppy for further experiments, BUT you don't have to agree to that just yet. The advantage of choosing this option is that you will be able to think your decision over and prepare to attack The Caller.
If you intend to betray Orthorn and "exchange" him for the books, you need to approach The Caller and inform her of your intentions (screen above). Collect the books afterwards. Note - Don't try to take the books before speaking to the Caller, as it would lead to a fight!
If you prefer to defeat The Caller in battle, you should try to attack her by surprise (screen above), so that she's defenceless for the first couple seconds.
The Caller will summon a few Atronachs to help her (screen above) and they might be of the Flame, as well as of the stronger Ice and Storm types. Try to attack them first, especially that new ones won't appear in their place. Also remember to use the proper elements, for example ice attacks to quickly get rid of Flame Atronachs.
Move to The Caller only after dealing with the Atronachs (screen above). The woman will keep teleporting throughout the whole room, but it shouldn't cause you too much trouble. Just keep an eye on the health level of you hero and the team members, attacking The Caller until you drain all of her health. Afterwards examine her body to collect the Fellglow Ritual Chamber Key.
Regardless of whether you fought The Caller or decided to give her Orthorn, you have to move on to the books (screen above): Fragment: On Artaeum, The Last King of the Ayleids and Night of Tears. Each of the books can of course not only be examined, but also taken (E). Note - If you did fight, the books might have fallen down, in which case you should use the in-game symbols to find them.
New main objective: Return the books
If Orthorn is still alive, you should talk with him to receive gratitude and separate. Use the north-west entrance to the main room. Examine the shelves and the chest. In the end use the hatch in the floor to return to Fellglow Keep (screen above). After reaching the new area, push the bolt and open the door to return to the room with stairs. Now you just need to head forward and open the main door leading out of the Keep.
Open the world map and fast travel to the College of Winterhold. There, head to the Hall of the Elements and afterwards to the Arcanaeum. Find Urag gro-Shub and speak with him to return the books. Urag will send you to Tolfdir, to inform him of the curious content of the Night of Tears. The mage will also rewards you for completing the quest, giving you six skillbooks:
- Racial Phylogeny - boosting Restoration upon reading
- Daughter of the Niben - boosting Alteration upon reading
- 2920, Hearth Fire, V9 - boosting Conjuration upon reading
- Catalogue of Weapons Enchantments - boosting Enchanting upon reading
- Response to Bero's Speech - boosting Destruction upon reading
- The Black Arts on Trial - boosting Illusion upon reading
College of Winterhold quest unlocked: Good Intentions