The door of the room burst open in a shower of red sparks and Hermione wheeled around as Professor Lupin came hurtling into the room, his face bloodless, his wand raised and ready. His eyes flickered over Weasley, lying on the floor, over the Slytherins, sitting on chairs watching, to Potter, standing there with his wand covering Black, and then to Black himself, crumpled and bleeding at Potter's feet.