Harry Potter and the Order of the Phoenix
by J.K. Rowling
The Bloody Baron
The Bloody Baron is the Slytherin ghost. He is a rather intimidating ghost covered in silvery, spectral blood. He seems to be the only person in all of Hogwarts (including Professor Dumbledore) who can keep Peeves the Poltergeist under any kind of control. That goes to show exactly how scary he must be.