Freedom for a Fiend

Quick Walkthrough
  

Detailed Walkthrough
By interacting with an NPC and asking for "latest rumors", you will be told that there is a strange noise coming from the  . Enter the basement vault of the Imperial Archaeological Society. Pick the lock on a small chest: there, you will find a soul gem, which speaks to you. There's a spirit trapped inside. The creature asks you to free the it and help it seek revenge against the warrior who imprisoned it.

It will direct you to the curator of the Imperial treasure collection,  . He oversees an ancient Dunmer blade that is necessary for you to free the spirit. Lloras can be found inside the vaults of the   palace.

You can try to steal the blade directly (by stealth or violence), or try to bargain with Lloras Nelvel for it. If you decide to negotiate with him, Lloras will ask you to kill a Tribunal Temple priest named   accused of Royal slander. He is at the  , which is east of   and the  . Keep in mind that once you make the agreement, you can't go back; you will have to kill the priest to continue the story unless you use the console commands. 

Once you've killed the priest and Nelvel knows of it, he will leave the blade in an out-of-the-way spot for you to pick up. Go to the top floor of the Tower Curia in Ebon Tower. The sword is in a long closed-off storage room atop a crate. After taking the sword, the strange soul gem will fuse with it. Now you will be commanded to kill  , who resides north of  . 

Ask around Seithur and the locals will tell you Bartolomaeus has been seen fishing along the coastline north of the city. There he stands, quietly fishing near the several mudcrabs, well north of Seithur and the monastery you'll pass along the way. When you meet him, he is aware of what is going on, as he senses the malignant presence of the spirit. He explains that you are dealing with the Fiend; an entity which forcefully occupies and takes control of other organisms and that he indeed was the warrior who sealed the spirit away. Although defeated the Fiend, he was unable to correctly seal its soul due to his inexperience at the time, so the sealing was defective. The Fiend has planned to get revenge on Bartolomaeus, intending to steal his body by having you stab the warrior with the ancient Dunmer sword that defeated it.

You now have to choose to aid Bartolomaeus in permanently defeating the Fiend, or to side with the Fiend in exacting its gruesome revenge. 

Siding With Bartolomaeus
If you take the warrior's side, the Fiend will immediately turn on you both, knocking Bartolomaeus unconscious, leaving you have to fight the Fiend alone.

The Fiend will successively take on the form of several powerful monsters. In the end, it will become exhausted and attempt to flee as a scrib; don't take pity on it, just kill it. Once the Fiend is dead, Bartolomaeus will give you his Signet Ring as thanks.

Siding With the Fiend
Siding with the Fiend will lead to battle with Bartolomaeus, who despite his humble appearance is actually a very powerful wizard. 
He will immediately summon two Skeleton War-Wizards to aid him. You will face very damaging skill and attribute-damaging spells and poison attacks. Even powerful characters with strong magicka-resisting armor and items will face a challenge here, so be aware that he will not go easily. 

After defeating Bartolomaeus, interacting with his body will give you the prompt to impale his body with the cursed Dunmer blade, enabling the Fiend to inhabit his enemy's body and bind Bartolomaeus' soul to power your reward, the powerful enchanted Daedric trident, Contempt. 

The Fiend will then disappear after a few unfriendly parting words...

 
 

Quest Stages