On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for the King of the Hill side quest.
Rewards: 1,200 XP.
In the place shown in the above image, in a makeshift wooden fort, you will meet a man who is apparently waiting for a rival to fight.
Talk to him and then agree to fight. When it is all over you must leave the area and wait for the man to start boasting about himself. Return to the arena and teach him a lesson a second time.
To complete this task, you will have to return to the man a third time. After the fight, speak with the warrior and the mission will end.