Pentiment

You must accuse Brother Guy of the murder in act 2.

For that not all the following things have to be fulfilled but they help a lot:

• Be nice to brother Wojslav, so when you talk to him in the library in act 2, he will give you additional information.
• Talk to Sister Gertrude and Sister Margarete about whether they know anything suspicious about the second murder.
• Go to the sacristy and look at the door on the right to find a book.
• Go to the scriptorium to Guy's table to find that he writes with lemon ink.
• Go to the prior's house where you will find a candle with which you can read the letter with invisible ink (lemon), be careful not to burn the letter.
• Go to hear the confessions of the people in the church of Father Thomas.
• Go to Sister Illuminata for lunch and agree to help her. You must enter the library so be incredibly careful with the answers you give to the guard. In the library talk first to brother Wojslav and then to brother Guy, Important (Do not talk to Father Gernot before doing this, and if you do, DO NOT TELL HIM anything about your suspicion or you will block the following conversations with brother Guy).

It is possible that I am forgetting some things, but this should be more than enough to be able to accuse Brother Guy.