On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the side quest The Anchoress.
Rewards: 1,200 XP.
In the place marked in the above screenshot, you will find a brick crypt. The doors will be closed but you'll notice there is a lot of food spread around.
Go around the building, stand in the spot shown in the image above and shoot off the door lock, then go underground.
Downstairs you will find a nun. When you inform her that you have destroyed the lock blocking the door and she can go outside, the woman will reply that this is the life she has for herself. If you have level two Charisma, you can directly convince her to come out. Once you leave the crypt, the mission will end.