Silent Killing

As the name suggests, this technique is simply a very quiet method of killing an opponent, usually from behind with a blade. Additionally, because the user is very silent, it cannot be defended against. The user will often slit their opponent's throat, preventing any cry that might give them away. Zabuza was a master of this technique, and was even good enough to track and kill opponents through sound alone.

The Hidden Mist Technique was used by Zabuza in conjunction with this technique.

Trivia

 * Although the name of this technique is written in kanji (無声殺人術), it's pronounced in English ("silent killing"). The Japanese pronunciation of the kanji would be Muon Satsujin Jutsu, which would literally translate to "soundless murder technique".
 * It is possible this technique is associated with ANBU, as both ninja known to use it are former members of their village's respective ANBU (Zabuza Momochi and Kakashi Hatake).
 * According to Kakashi, this technique allows the user to kill their opponent so quickly, that they "pass through this world without even knowing what happened".