As soon as she admitted that she was the one he was looking for, he said: "Incarcerous." Cords shot from his wand and, if she did not manage to avoid them, would tie her wrists together, while another cord would wrap around her body, pushing her arms against her body and probably causing her to lose her balance. Arthur wouldn't take any chances with this lot. She was almost certainly a vampire, and a vampire who had committed a brutal murder. He wasn't going to stand her and wait until he ended up like poor Mr. Baudelaire.