Received from: Aedirnian peasant (M15, 4) during Royal Blood.
Step 1: Talk to the suspect peasant
The peasant suspected of poisoning Saskia can be found in the southern part of the city. Willy Oblate will be guarded by a Scoia'tael squad (M15, 19). For the peasant to start talking, you have to use Axii hex or intimidation. You will learn that he overheard priest Olcan talking with the prince.
Step 2: Find the priest's house. Ask Cecil Burdon for help
In order to gain information on the location of the priest's house, it's best to head to the elder - Cecil Burdon (M15, 12) - who will show you where Olcan's house is on the map. Head there (M15, 20) to find quite a mess inside. Focus on Olcan's notes lying on the table.