On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the King of the Hay People side quest.
Rewards: 1,200 XP.
In the place marked on the image above, you will meet a powerful man sitting on a straw throne.
Just eliminate this elite opponent and take the treasure from the chest on the left, and the mission will be over. To make this task a little easier, use the Hidden Blade.