This involves what you tell Archdeacon Jacob during your questioning at the end of Act 1.
If you decipher the entry in the book Ferenc writes in (from obtaining the achievement Like a Record Baby)
And convincing the Abbot to dig in the grave (from obtaining the achievement Grave Matters)
You have enough evidence to convince Archdeacon that Ferenc is the one guilty of the crime.
Just make the dialogue selections showing your proof why you think it was him and finish up the questions (to ensure blame, do not give info on any others) and when you end up at the Town Center after the cutscene, the achievement pops.
Happy hunting!