Here’s a quick quiz: What are dementors? What shape does Harry’s patronus take? Who is Severus Snape, and which actor played him in the films? What is Remus Lupin’s affliction? What is the name of Rubeus Hagrid’s favorite pet? Name one of the balls used in quidditch. If you could answer even half of those questions, then this is for you: To promote its showing of all eight Harry Potter movies for a six-month run, HBO is holding a Hogwarts House Challenge at the Blake Street Tavern on Wednesday, Jan.