On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for the The Puppeteer side quest.
Rewards: 1,200 XP.
In the place marked on the image above, you will come across the corpse of a man, which surprisingly can talk.
The corpse wants you to donate a small offering (10 pieces of silver) so that it can go peacefully to the land of the dead. Of course, this is just a scam which you'll reveal in a moment.
Climb the rock next to the man's corpse and enter a nearby house. Once you're inside, push back the heavy bookcase to reveal the entrance to the cellar.
Go downstairs and you will come across an imposter in a narrow hallway. Eliminate him and the mission will end.