Pentiment

Church, go right, go down to the crypt, go right to the end, click the picture on the wall, click the key held by the baby in the picture

At the Abbey, enter the Church. Continue to the right and enter the crypt door in the background. Once in the crypt, continue to the right again and interact with the relief of the Virgin Mary. Click on the key in the baby's hand. This will open the secret door to the library and unlock your achievement.