Walk This Way - S1.E18

Cedric uses a magic horn called the Horn of Hypnos to make everyone who hears it into obedient zombies called trancemarchers. Unfortunately, Guardians of the Veil, namely Hay Lin, are no more immune to its effects than anyone else. How are the girls going to break Cedric's hold on their friend and what is being done about Phobos' Book of Secrets that Caleb swiped from their last trip into Meridian?