On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Jotunheim side quest: Pit of Slaughter.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will find an arena. Talk to the mighty Jotun (Dugr the Bold), choose an animal to change into (it doesn't matter which one), and join the competition.
Defeat the two shape-shifting enemies in a fistfight, then talk to Jotun to complete this quest.