What does Harry do to ensure that his friends are safe from the Nightmare's attacks?

Harry casts a spell to summon the Nightmare using its name, which he learned from Kravos' leather bound book. While he has the demon under his spell, Harry binds it to himself, so that it must attack only him until one of them is dead. Michael doesn't think the binding is a good idea, but Harry doesn't tell him until it is already done.