A Boggart's greatest fear

Harry Potter and the Prisoner of Azkaban

It’s always best to have company when you’re dealing with a Boggart. He becomes confused. Which should he become, a headless corpse or a flesh-eating slug? I once saw a Boggart make that very mistake – tried to frighten two people at once and turned himself into half a slug. Not remotely frightening.

Remus Lupin