This page of the Assassin's Creed Valhalla guide has a detailed walkthrough of The Devout Troll side quest.
Rewards: 1,200 XP.
Go to the place marked in the picture above – you will meet a monk. You will find out that there is no anger in his life at all – he won't get angry even if you burn down his farm.
You should check this! Burn down the house, smash the wooden crates, and kill the cow – you will manage to make the monk get up from the stool he is sitting on.
As soon as the monk gets up, take the key on which the man was sitting (Monk's Shed Door Key).
Go towards the river and open a small shed on the right. It is full of human bodies and remains.
Talk to the monk, and he will attack you. Defeat him to finish the task.