Head to the South from the Vineyard and through the archways. This will lead to the second part of the Monastery. Head over to the Monk who is standing by the group of people and talk with him. This is Friar Honore.
He is the Guilty Party.
Head over to him and hold the prompted button that appears above his head. You will then accuse him of the crimes that have been found at this abbey.
"The conspirators were foretold to die in their sleep. Jean was victim of fratricide. Is it murder to fulfill a prophecy?"