This page of Assassin's Creed Valhalla guide has a detailed walkthrough of the Degolas the Beautiful side quest.
Rewards: 1,200 XP.
In the place marked on the picture above, you will meet a family that has a problem with unpleasant smell in their home. After speaking to the boy, you will find out that it has to do with his father, who never washes, and the stinking chests with special arrows.
Before you enter the hut, use Odin's Sight to locate the stinking chests. Then go inside and move all the chests outside – you can throw them, they will not break.
If you stay in the poisonous area for too long, Eivor will start to receive damage. Fortunately, a lot of fruit grows around the hut – you will be able to quickly restore the lost health.
When you take out all 4 chests, the air at home will be cleared. However, this isn't over yet. Talk to the boy and then stand near his father.
Since the man doesn't want to take a bath, you must help him. Aim at the platform on which the boy's father stands and then shoot it – the man will fall into the water. The problem will be solved. Talk to the man to complete this quest.