This page of the Assassin's Creed Valhalla guide describes the Plight of the Warlock side quest.
Award for completion: 1200 XP.
In the place marked on the picture above, you will find a hunter who is apparently talking to himself while hiding behind the bush. Talk to him to activate an additional event.
The hunter will ask Eivor to help him kill the Warlock, who allegedly poisoned his sister. You will also find out that King Harald himself has set a reward for this. Follow the newly-known hunter to the Warlock's hut.
You can talk to him, but during the conversation, the hunter will be provoked and attack. Help him eliminate the Warlock to complete this quest.