I've identified the most frightening creature in the world of Harry Potter. Prove me wrong.
This creature is only referred to in hushed tones by those who survived a confrontation and is simply referred to as "Update". Updaticus Chaotica appears to be a vile cross-breed of a boggart and a Dementor. It is able to both identify your worst fear, while simultaneously sucking all joy from your life.
The only known protection from Updaticus Chaotica is the counter-curse, Uninstall.