B is for Boggart
Boggart: a creature which manifests into the witch or wizard's worst nightmare, like above. Snape is Neville's biggest fear. They inhabit dark places e.g. wardrobes.We encounter Boggarts in Harry Potter and the Prisoner of Azkaban for the first time, and I found them absolutely fascinating! The book and film provides some of funniest moments; from spiders wearing roller skates to Snape dressed up in Neville's grandmothers clothes. The reason for these hysterical transformations is that, in order to combat the Boggart, the witch/ wizard must imagine something which makes them laugh, casting Riddikulus in order to morph it into something less terrifying.
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.
- J. K. Rowling, The Prisoner of Azkaban.
Harry is not instantly allowed his turn at combating a Boggart, with the then teacher of Defence Against the Dark Arts, Professor Remus Lupin fearing that the form of Lord Voldemort would appear within the classroom. It was so interesting to see what a select few of the characters feared. Lupin feared the full moon (werewolf), Hermione feared failing, Ron feared spiders, Neville feared Snape and Harry, surprisingly, feared Dementors much more than Lord Voldemort. It is the introduction of this creature which leads Harry to his learning of Expecto Patronum.
For more information on Bogarts, click here.