In Assassin’s Creed Valhalla, after you set out to form an alliance with Oxenefordscire, the Pilgrimage To St. Albanes mission starts where you have to locate Fulke. The mission can be a bit confusing for many since entering the monastery is a bit tricky and Fulke is hidden inside a locked crypt.
To reach the monastery, you need to first head to the objective marker on the map. You can use your longboat to reach the place since it is beside the river, and once you are near the marker, you can actually start a raid at St. Albanes monastery that is located in Linforda.
Once you have cleared the enemies along the perimeter of the monastery, you have to find a way inside. This is a bit tricky since there are no doorways to vents to get inside the monastery. You have to locate a window with tinted glass in the church. Shoot the window with your bow and arrow to break the glass, and then head inside.
Before you head towards Fulke, you will see a wall on the right inside the church. Behind the wall is a treasure chest, and you need to break it to get the treasure. Use the red clay pots near the church to break the wall and you should be able to get past the wall.
There will be a locked door on the left, inside the monastery which will lead you to the underground chamber. The key to the door can be obtained by defeating the Yeoman inside the church. Once you have unlocked that door you can head inside and you will find the prison cell.
The prison cell will be locked and you need to obtain the key from the priest near the door. Interact with him and you will be given three options to choose from. You can either pay him money, kill him, or use charisma to get the key. Once you have made your choice, you can open the door and free Fulke.