"How do they react to magic?" he asked, already whipping his wand through the familiar series of motions required for a Summoning spell. His OWL project in Transfiguration had been creating forms from dust and sand; it had been a while, but it was the first thing that came to mind and the spell itself was nearly automatic. He called all the dust from the hallway and the rooms beyond, from the tops of door frames and cracks in the wall, cursing Travis and the House-elves as the particles of grit flew past them for keeping the castle so clean.
There hadn't been anyone cleaning for at least a day, however, and they were in a castle filled with children. He managed enough substance to form a mārid, towering over the confused House-elves gathering at the opposite end of the corridor and spinning rapidly like a sand-devil from the desert. William let it gain slightly more momentum, took five more stumbling steps backwards until he reached the intersection of another hallway, and then set the mārid on fire. It shrieked, spinning faster and faster as the flames consumed it, and William ducked around the corner just as the dust and dirt exploded outward in a blinding hail of ash.
He could still hear groans and shambling noises behind them, but he hoped that was because the House-elves couldn't see and had quite possibly caught on fire rather than because they were still in hot pursuit. He didn't really want to hang around and find out.
"Distracted," he panted, catching up to Shane. "I don't know for how long."