He Who Must Not Be Named
Most Commonly used to refer to Voldemort of the Harry Potter Series, but also used to describe someone when you don't want to say their name out loud- in case somebody else should hear you.
Friend 1: Who's that guy you hate?
Friend 2: (points) Him, over there, but don't say his name!
Friend 1: Oh, okay. I hate HWMNBN, too.