Castti <!chap 3 & 4 about “him?”>

So I just finished chapter 3 and four for this character and the guy who turned on the group kept mentioning someone “who opened his eyes to the truth” and I haven’t done the third chapters for anyone else yet since I liked this character a lot and I heard you can get an item from a quest after finishing this story and so I wanted to discuss who is him and what did he say to make the most kind hearted person in her group to turn evil and start killing everyone with poisoning