Madam Magician meant: the girl in the coffin underwent a transformation the instant she simultaneously received the powers of a Demoness of Catastrophe and an Undying through a strange ritual. This corpse, now possessing Angel-level power, was a monster from the moment it came into being.
Lumian, in his Demoness state, frowned and said, "Aren't boons supposed to not affect the spirit, only gradually causing mental and physical alterations?"
Surely it shouldn't directly create a monster?
Moreover, from what he could see, the powers of the Demoness and Death pathways weren't so contradictory and could be integrated to some degree.