This is a very simple achievement if you know how to start it.
First, in Paris, go to the magnifying glass on the map. There you will get a short cutscene where a prisoner is complaining about unsolved murders. When the cutscene ends speak with the policeman and the prisoner and he will give you a mission to solve one of these murders. It has been confirmed that the first mission is to investigate the death of some monks.
The mission is marked by a green dot on your map and you must go investigate a monastery. Use your eagle vision by pressing
and you will see evidence highlighted in yellow. There are 13 items you must investigate in order to get 100% on the investigation. (You do not have to do this, you can skip right to the criminal and accuse him if you want, scroll to the bottom and I will tell you who it is.)
Investigate:
Upstairs first!
1. The dead man on the floor
2. Dead man on the bed #1
3. Dead man on the bed #2
4. Dead man on the bed #3
5. The living man near the railing
6. The crumpled piece of paper on the floor
7. The Chest in one corner
8. The second chest on the floor
Downstairs
9. The living monk by the fireplace
Outside near house:
10: The gardener in the garden
11: The fire on the other side of the garden away from the house
Outside to the right of the garden
12: The monk by the archway
13: The monk standing in a group speaking (accuse)
The final monk will be ranting about Nostradamus and prophecies. This will be the person you want to accuse. After you speak with him, a yellow dot is put above his head that says "Accuse." Speak with him again and then hold
to accuse.
That is it, achievement unlocked. It is the same every time, you can skip all of the evidence and go straight to the monk (farthest away from the house, ranting about prophecies) and accuse him.