& Cassie is all dark magic because her dad & the guy said she's not the only child of Blackwell in the circle. I'm thinking it's Faye because they both seen Faye's younger self, Faye's mum had a major crush on Blackwell so they probably did it...
Plus Faye is always craving more magic so it would be a good set-up to turn her character into a power craving psycho for a bit until the series finale where she is saved by Cassie