"Do you really wish to know?"
Spoilers from the books and/or adaptations to follow!
Willy Oblate or Willy of the Oblates was a servant of Saskia. He was one of the main suspects when Saskia was poisoned, hence was forced to hide out of Vergen, where was guarded by Scoia'tael.
If Geralt talks to him: Willy will reveal to the witcher that Reverend Olcan held a important part in the poisoning.