This was my initial though; that Edgar Neverever is the one who killed Dilton's father and it is his daughter who is trying to use it but is either inexperienced or has the dose wrong and is just causing seizures in the people who she is trying to hurt.

Whether Edgar is the Gargoyle King or not I'm not sure, but we know that he runs the farm, and that Alice has now sent Betty to the SoQM and they are already under the spell of the Gargoyle King, both indicated by the paintings of the other wards of the sisters but also what the nun said to Betty about "I know you've looked down on our practices in the past, Ms. Cooper but I can assure you.. we're going to make you well."

Isn't that really similar to what Edgar's daughter and the other farm freaks have been telling Betty?

My other theory is since the daughter tried to get Betty to join her farm circle based on the treatment of her seizures it's possible she's intentionally just causing seizures in people? I don't know.