The Killing Curse

One of the three Unforgivable Curses outlawed under wizarding law, Avada Kedavra kills the victim on impact, instantaneously and painlessly, usually with a blinding flash of green light. There is no way to block the spell, and there is no counter-curse - it can only be dodged. Harry Potter is the only known survivor of the Curse, having been protected by Lily Potter's sacrifice - the spell instead rebounded upon the caster, Lord Voldemort. The Curse leaves a victim's body completely unmarked, with their final expression frozen on their face - this can perplex any Muggles who discover anyone killed this way, as in the case of the Riddle family.