Notice! This mission will be cancelled if you accept Retaking Thirsk!
When you approach Thirsk Mead Hall, you should be approach by one of the monsters and asked to follow him. Enter the building to meet the Riekling Chief, who - surprisingly - can speak human. The resolute leader will ask you to help him with some of the most problematic matters.
Keep in mind that the goals of Rieklings are completely different from the ones of Nords - by helping one side of the conflict, you will automatically irritate the other.
Give Bilgemurk some meat
The first part of the mission is relatively easy - you need to find the beast known as Bilgemurk and feed it. If you don't have any meat, hunt down some horkers which can be found lying on beaches of Solstheim - that should do the trick. Afterwards head north of Thirsk Hall (southwest of Skaal Village) and approached the creature, which will turn out to be a boar. Give him some of the meat - you can do it by starting a conventional "conversation".
Return Bilgemurk to Thirsk
After feeding the animal, it will start following you. Return to Thirsk Hall and head towards the small tents in its southern part. Bilgemurk should head into his croft on his own. Apparently the beast has been somehow tamed by the Rieklings and serves as a guardian.
Talk to the Chief
The Riekling Chief will be happy to learn of the return of his favorite pet. That's however not the end of the attractions that he has prepared for you. It turns out that the village ran out of a plant called scathecraw. It's a very important ingredient, crucial for religious ceremonies of Rieklings. You have to obtain 10 pieces of the plan and bring it back to Thirsk Hall.
Bring 10 Scathecraw to the Chief
You're lucky - scathecraw is by far the most popular plant on Solstheim. It can be found in the southern wilderness of the island and at a few alchemists (one lives in Raven Rock and the second on in Tel Mithryn). Obtaining ten pieces shouldn't cause you any trouble - just look for large red bushes growing on the ashy soil. Also note that apparently the whole of Raven Rock is covered with it - it's likely to be used as decoration.
Kill the Nords
After giving ten scathecraws to the Chief, he will give you the last mission - you have to kill the Nords who want to recapture Thirsk Hall. You will be accompanied by a small group of Rieklings - they are quite good warriors, so fighting the weakly equipped soldiers shouldn't cause you any trouble (you however cannot hope to solve this matter quietly - trying to sneak will fail). The Nords can be found in a small camp in the east, known as Bujold's Retreat.
Speak to the Chief
Before you manage to walk away from the battlefield left from the Nord camp, you will be approached by the Rieklings Chief (who bothered to come to you personally). The old dodger has figured out that you're too powerful and will likely want to take over Thirsk Hall (and therefore become the new chief). There is only one solution of this uncomfortable situation - and honorable duel!
Defeat the Chief
The fight with the Chief won't be too long - he's not much more powerful than ordinary Rieklings. Even though he can deal quite some damage with his spear, he's delicate enough for you to crush his bones easily. A few good attacks should solve the matter in a brutal and final matter.
The death of the Chief ends the current mission. Note that by winning in the duel, you gain control of Thirsk Hall - what's more, all the villagers became you servants. You can ask any of them at any given moment to accompany you. That's however not all! You can hope for help from your servants as you're fighting monsters throughout Solstheim - there's a chance that a few of them will appear out of nowhere and fight for you.