The Deathly Hallows

"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."
—Professor Lupin to his Third anno class in 1993[src]

Boggarts are particularly fond of inhabiting certain places that are dark, such as in wardrobes, the gap beneath beds, and the cupboards under sinks and desks. On at least one occasion, a boggart was found hiding in a grandfather clock.
A boggart...