On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough description of the quest A Prayer for Vengeance side quest.
Rewards for completing the mission: 1,200 XP, silver pieces.
In the place marked in the above picture, you will come across a cave, the entrance of which has been blocked with a heavy bookcase. Only after you get inside will you be able to complete this side quest.
Stand in the place shown in the image above. Aim for the wooden box blocking the bookcase, then return to the main entrance, move the obstacle and go inside.
Inside you will hear the voice of a mad monk who says something about the crimes at Chertsey Abbey Ruins.
Exit the cave and you will be attacked by a deranged man. After the fight, the mission will come to an end.