Reiki (零毀) is a nodachi in the manga and anime series Tenjou Tenge passed down through the Natsume Family.
Reiki was created by Iya Natsume using her Ki just like Aya did with her hair when she created a false version of the Reiki. After it was created Iya's mother Himiko Amaterasu imbued her Amaterasu’s Dragons Gate, that nullifies all special powers, into the sword. In the present it is considered a ceremonial Nodachi, and a heirloom of the Natsume family. Despite its length classifying it as a nodachi, the characters tend to refer to it as a katana. It is very long, with a red sheath and hilt wrap. It has no cross-guard, and the sheath typically has a tattered cloth spiraling down it. In the past it was used to entrap all the power of evil Buddhist ceremonies. Reiki has the ability to enable people with Red Feather powers to use their powers more freely and enhance them. Reiki is vastly influenced by the nature of the wielder, its powers can vary from making it rain to causing the wielder to go on killing sprees.
It was given to Maya Natsume by her father, Kaima Natsume, at the age of eight to acts as its guardian, since she only current member of the Natsume family with no Red Feather/supernatural powers. However, Reiki's spirit told Aya Natsume that eventually the two will merge together and be the katana for the person who will break the chains of fate. This seems to indicate the possession of Reiki has been to Maya Natsume from the very beginning.
Reiki actually has a spirit of its own and can manifest itself to those have the power of the Dragon's Eye. Reiki is generally blamed for Shin Natsume's insanity and although the sword is typically considered evil by the characters, its spirit is not depicted as having malice. Reiki only makes awakening the power take less effort. Given the nature of the Dragon's Gates, this often left the wielder to fight with their own power. This combination of Reiki and his power often led Shin to go crazy. However, according to Fu Chi'en, Reiki had also been helping Shin to suppress his powers.
Reiki’s power even has Sohaku Kago apprehensive of it, as Tetsuhito Kagiroi states no one is a threat to Sohaku if they don't have Reiki. Mitsuomi Takayanagi also dislikes it and wants to destroy it because he feels it brings death.
Reiki's power, simply defined, is the same as the Amaterasu Dragon's Gate, neutralization of power. This is because Himiko Amaterasu, the first owner of the Amaterasu Dragon's Gate, had her gate imbued into the sword. This allows Reiki to consume large quantities of ki, and render those with Red Feather powers as nothing more than "dolls" as Madoka Mawari states.
Reiki can only be activated by those with special powers,those with Red Feather blood, and has even awakened powers in some cases. When wielded by someone without special powers it is nothing more than a very long katana. If the wielder establishes a good enough relationship with Reiki, it can even be activated while the user is not in possession of it.
When activated the blade allows the user to control and regulate their power better. The sword gains a visible aura around it and emits a piercing sound usually written as a "RIIIIIIII" in the manga. While activated the blade also becomes encircled with energy rings with kanji on them, strange symbols appear on the blade as well. While activated Reiki becomes more powerful, even if it can't cut the opponent, they can be burned by the energy it emits. When Reiki true blade is completely activated by the Dragon's Eye, it uses an excessive amount of ki and can wear out most users out in a short period of time.
The reason that the Reiki can do this is because of resonance. All gates resonate with each other, to varying degrees and by resonating with other gates it makes the power easier to control and more reliable so just being in the presence of someone else with a power makes your own power easier to use. Even though Reiki has the power of neutralization gates resonate even if they’re not in use. The Amaterasu Dragon’s Gate also has the most resonance of any gate, plus its contains the potential for amplification;? so resonating with it lets the user fully utilize and control their power. This big resonance also makes touching it awaken dormant Dragon Gate powers as seen with Shin.
For most of the series, Reiki was encased in gold and that functioned as its blade. Not until this casing was shattered by Tetsuhito was Reiki's “true blade” finally recognized.