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If she lives she blows poison mist and vanishes as she runs through it.
I chuckle every time I see this guy, I half hope her name is genuinely pronounced "Cuck-you-lus". I'm sure it isn't, though, and that bothers me; does anyone know how it's meant to be pronounced? Has a dev or a writer ever mentioned them in an interview before? If I can't ruin the illusion, the Age of Immaturity shall be perpetuated... By my hands.
The easiest way to get her to survive is to go full magic build with 60 int, use Great heavy soul arrow and spam it from near the door way, he dies pretty quickly since he's a demon and she never runs in to die like the tardlord she is.