Jeremy isn't fully inside when Matthew Fier makes his move. Matthew raises his amulet over his head which makes the younger man pause. He knows something is up but not what. Matthew yells out "Dominatio per malum" which are the same words on the back of the amulet.
This causes Jeremy's head to explode! What the heck? At least I'm not the only person with a shocked reaction as everyone but Matthew is confused. Even though the whole thing is shocking, Mary was the only one to figure something was up.
A nice bit of body horror later and another head has replaced Jeremy's. Specifically William Goode's. It's revealed that Jeremy never existed and it was William the entire time! I admit to being impatient with reading this book that I...um...skipped ahead...anyways...
William and Matthew trade some 'witty' dialogue like "But your powers are stronger than mine." Though their lines are league better than Mary who has turned into an utter moron. She asks what happened to Jeremy when it's extremely clear there never was a Jeremy. So Matthew has to tell his daughter what's obviously going on.