This page of the Assassin's Creed Valhalla game guide includes the walkthrough description of The Silver-Tongued Traitor side quest.
Rewards for completing the quest:
- 1 200 XP;
- 30 pieces Silver;
- Ball of Yarn.
You will meet Sulke upon reaching the spot indicated in the screenshot above. As it turns out, this sneaky trader wanted to make money by selling things belonging to Eivor's family.
As soon as you expose his shady business, Sulke will run away. Your task is to catch the man and kill him. When looting his corpse, you will find a key to the chest and some Silver.
If you're prepared for Sulke to start running, you can attack him with a light attack before he starts running. Your opponent will be eliminated on site and you will save some time.
Use the key found on Sulke's body to open the chests under the shelter. Inside you will find 20 pieces of Silver and Ball of Yarn.