On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for the Madness of the Stones side quest.
Rewards: 1,200 XP.
In the place marked on the above picture, by a small stone circle, you will meet a crazy man.
When you talk to him, the man will ask you to count the stones. When he counts, he gets a different result all the time. There should be 21 of them during the firs try. You will have to repeat this action two more times, but you will not be able to get the same result.
After a while, Eivor will lose consciousness, and when he wakes up, there will be no trace of the crazy man - a large stone has taken his place. This will complete the mission.