On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough description of the King of Shitsby side quest.
Rewards: 1,200 XP.
At the location marked in the image above, you will hear noises coming from the old church. Go inside, examine the strange throne and sit on it.
After a moment, an unfamiliar voice will speak up. Choose any dialogue option to provoke the man. The place will be overgrown with bandits, whom you will have to defeat. After the fight, the mission will come to an end.