A dementor is a man-size creature in the Harry Potter books. They are black and hooded. They only lift up their hood for performing the kiss in which they suck out the soul. There is one way to make them go away. It's the Patronus Charm. It's very advanced magic. You perform it by saying Expecto Patronum! and remembering a very happy memory, a very powerful memory. Harry Potter first saw them in the third book The prisoner of Azkaban.