This page of the Assassin's Creed Valhalla guide has a detailed walkthrough of The Walloper side quest.
Rewards: 1,200 XP.
Go to the place marked in the picture above – you will encounter fighting children. One of them claims that nobody can hit as hard as their grandfather, the famous fighter.
Talk to them, and after a while, their grandfather will come. During the conversation, you can decide to fight and see if the children are telling the truth.
Note! One accurate hit will knock Eivor to the ground and finish the duel. Direct attacks won't damage the opponent. You have to block his attack perfectly to knock the enemy to the ground – you will finish the fight with one punch.
After the fight, the man will give you a key to his house with a treasure inside. When you enter the hut, collect the item marked in the picture above to complete this task.