Bloodmoon:Tymvaul in the Well

Quick Walkthrough
# Talk to Lassnr in the Skaal Village and receive a well key.
# Follow the tunnel in the well and enter Rimhull.
# Find Tymvaul and decide how to handle the situation.
# Return to Lassnr.

Detailed Walkthrough

A Worried Father
Inside his house in the Skaal Village, Lassnr will ask you to find his son, Tymvaul, who recently fell into the well. Beneath the well there are the Rimhull caves and he hopes his son is somewhere there still alive. Agree to help and you'll receive a well key to give you access to the well and Lassnr mentions that there is probably another entrance to the caves if you can find it.

A Passage in the Well
The well is just behind Lassnr's house to the west and there is another entrance to Rimhull a little to the south of that entrance, down the hill and close to the river. If you enter by the well entrance you'll find yourself underwater on the other side, but it's a short swim to exit the water. The cave is populated with a number of undead, mostly skeletons, which are not too tough if you can fight them one at a time.

Decisions, Decisions...
Shortly after entering, you'll find Tymvaul, wearing a purple robe and definitely not happy to see you. He chatters something about the Mantle of Woe. It seems Tymvaul has heard legends about the magical robe and ventured into Rimhull in order to find it. You are given five choices in how to handle the matter:

#If you tell him that he's insane and must die, you will obviously fight with him. After you killed him, you can tell the truth to his father or lie. Telling him that you have killed his son, will make him attack you too. Lying instead, saying that you've found his son already dead, will make him sad (but will increase his disposition to you) and he will reward you with two snow bear pelts.
#If you tell him that you are sent by his father and try to convince him to take off the robe, he will understand that his father truly loves him, he will give you the Mantle of Woe and tell you that he will leave Solstheim in order to study magic better. Reporting the good news to Lassnr will make him extremely happy (again increase of disposition) and he will reward you with five snow bear pelts. This will also make Tymvaul disappear from the game.
#If you tell him to join forces and bring death to Solstheim together, he will refuse and attack you. Once again, if you tell his father the truth he will attack you too. Lying instead, will make him reward you with two snow bear pelts.
#If you tell him that this is not your business and you're going to leave, he will try to stop you by attacking you. Once he's dead, again, you can tell the truth to his father, causing him to attack you too, or lie and receive two snow bear pelts.
#If you tell him that he's too weak to wield such power and that you want his Mantle, he will of course attack you. As with the previous options, you will be able to tell the truth or to lie to his father.

The "good" and best option is obviously the second one, convincing Tymvaul to give up the robe and also being rewarded with five snow bear pelts (useful to craft armors). Any other option will have you kill Tymvaul and get, at max, only two pelts. Either way, you get to keep the very interesting magical robe.

Notes
It seems Tymvaul was doing some experiments in the dark arts before he entered the well. In Lassnr's house, you'll find a basket of skulls, some interesting books on necromancy and other dark subjects, as well as some very rare ingredients: ghoul heart, human flesh, and wolfsbane petals (this is the only place to find wolfsbane other than a single plant atop Hvitkald Peak).
If you encounter and kill Tymvaul before receiving the quest, Lassnr will know of his death (though will not seem to blame you for it), and will have no dialogue other than his grieving. 

Quest Stages