Description
A tall hooded thing with no face under the hood, gliding a hand's breadth above the ground. Where it goes the air turns cold enough to see breath, and every good memory in the room goes out like a candle. It feeds on hope, and what it leaves behind is alive and no longer anybody.
Background
The Ministry removed the dementors from Azkaban two years ago and has not been able to say since where a great many of them went. The only known defence is a Patronus, which drives one back; nothing kills them.