I did it about a week ago and then again yesterday with someone else. The quest came from the King (I think, I have helped with a lot of quests, so dont recall, sorry) Dont know if you HAVE to do it. but warning, if you do it, make sure you have some friends with you as all the foes are very powerful.
When you enter Nidalvir, you will see three different dungeons. The middle one is Hall of Valor where Jotun resides. The other two belong to his brothers. By defeating his brothers in their separate dungeons and then going into the Hall of Valor, you will only have to face Jotun and one minion.
Here are a few tips about these dungeons: 1) you do not have to do them all in the same day.
2) You don't need the same group of people with you for all three dungeons. As long as YOU defeated them and enter the Hall of Valor alone or with others that defeated the brothers Jotun will be alone.
3) If someone who hasn't defeated the brothers enters the dungeon with you, then you cannot pass the door that is up and to the right of the entrance. Doing so will trigger Jotun's brothers to make an appearance.
4) If you pass the door up and to the right of the entrance, someone who hasn't defeated the brothers can then port to you, but you CANNOT go back through the door, doing so will trigger the brothers.
5) If you pass the door up and to the right of the entrance without getting a challenge, then Jotun will be without his brothers. If you do hear a challenge, then one or both brothers will be there.
These are just a few of the things I learned during my many hours and days of farming those dungeons for pets and a certain furniture item.