Shinigami Blade

The Shinigami Blade is the sword held in the mouth of the Shinigami.

Uses
Shinigami: uses it to cut the souls so they can transferred into him

Maomi Hōzuki: uses it to provide proper defense when she is using her Curse Mark.

Trivia
Even though humans can use the sword, it it only be used using the Shinigami Curse Seal, due to only the spirit's chakra being able to hold and use it.