The Prisoner of Azkaban
THE MARAUDER'S MAP
¡°Couldn't have put it better myself,¡± said Professor Lupin, and Hermione glowed. ¡°So the Boggart sitting in the darkness within has not yet assumed a form. He does not yet know what will frighten the person on the other side of the door. Nobody knows what a Boggart looks like when he is alone, but when I let him out, he will immediately become whatever each of us most fears.