On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Jotunheim side quest: The Puppeteer.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet a Jotun who needs help. A hunter living in a cabin in the woods went crazy, has beaten her beloved one, and locked him in a cage. The woman asks you to help her open the cage.
You must deal with the Jotun illusion while performing this task. Break the lock of the door shown in the above image and go through it. Turn around, wait a moment, and go through them again to find yourself inside the hunter's hut.
After a while, the hunter will run down the stairs, and a fight will start. When a man gives in, you can decide what happens to him. Take the key, go outside and release the prisoner and the mission will end.