Place it at the very end of a reveal or defeat, after the setup has finished, then follow it with a fail sound to let the moment settle. Pair it before a dramatic sting for a villain entrance, or after a knowing setup line for a twist ending.
About this sound
This short clip, just under five seconds, has Alastor Moody delivering the two-word Avada Kedavra incantation in a low, gravelly and forceful voice, clipped and final. It feels menacing and tense, instantly signalling death and a villainous turn without any explanation. It comes from Harry Potter, where Moody demonstrates the Killing Curse while teaching Defence Against the Dark Arts.