The Grim Reaper had come to Hogwarts.  He stood an impressive 4'4 inches tall and was shrouded entirely in black cloth that he'd worked very hard to age with both magic and a bit of burning.  Nothing was quite as fun as setting things on fire and the detention he'd gotten for it was totally worth it because he.. was... awesome.  His robes looks aged to the point that they were like layered black cob webs hanging from his skinny shoulders.

Grim had a hood of the same material pulled low over his face and beneathe that, a death mask that was terribly cheap plastic but he didn't care because the real show stopper of his costume was the black, smoke-like haze that seemed to seep from the bottom of his robes.  It followed him when he walked and the boy was sure right then that he had the coolest mom that every lived; she had known just what to do to take it to the next level.  

So, trailing his essence of death and grinning like a fool beneath his mask, the Grim Reaper took up his costume-shop scythe and wandered into the Great Hall to strike fear into the hearts of all!