Tysyacha

He's smart enough to make himself immune, but dumb enough to unintentionally allow Lenatha to find the means to do so as well. But he makes up for that by crippling the group with fear and doubt, and turning some of them against others.

PS: Like what I've done with Iyav? How different is he from your original idea?

PPS: I only ask because I've been reading A Game of Thrones, and Knights are called Ser rather than Sir, like in Dragon Age.