Killing Curse

  There was a flash of blinding green light and a rushing sound, as though a vast, invisible something was soaring through the air — instantaneously the spider rolled over onto its back, unmarked, but unmistakably dead  

The Killing Curse (Avada Kedavra) is a tool of the Dark Arts and one of the three Unforgivable Curses. It is one of the most powerful and sinister (if not the most sinister) spells known to Wizardkind. When cast successfully on a living person or creature the curse causes instantaneous and painless death, without causing any injury to the body, and without any trace of violence.