A Death Note is a supernatural notebook used by Shinigamis to kill humans, so they can extend their lifespans.
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Origin: Death Note
Range: Multi-Universal (Can kill humans on Earth from the separate Shinigami Realm)
Weaknesses: The wielder must know the victims' name and face, the Death Note cannot make the victim do the impossible, it cannot kill a shinigami, and is only known to work on humans.
Note: A full list of the rules of the Death Note can be found here.