The Narchuriki (Power of Human Death and Negative Energy Animal Users) are a race of humans that have a Tailed Demon Immortal seal
Power of Human Death, Reincarnations of Darkness, Negative Energy, Animal Users
|Main Power Source||Tailed Demon Immortals|
|Primary Seal||The Mark of a Nachuriki|
within them. They exhibit negative extraordinary powers, in any case, when a person dies or almost dies, he or she is given a chance to live by becoming a Nachuriki at the time of death. If they display a will to live or a reason for revenge, a Tailed Demon Immortal may spare and turning them into a Nachuriki in the process. Once a Nachuriki is create, he or she can't no longer be a part of the world they use to be.
History (Soul Reaper World)
The Nachurikis were once a race that exist in the Soul Society, in a divded land with the Jinchurikis, the Nachurikis use ancient negative powers and human experments. The Jinchurikis saw the terrors and crimes, the Nachurikis have done, so on one night, the Jinchurikis decide to fight back against the Nachurikis, saving their prisoners, and to destroy them to extinction. This battle was called and known as "The Light vs Darkness War", as the Nachurikis lauch every negative powers they got, even after they lauch every negative powers they master, they won't a match against the Jinchurikis with her Jinchuriki Tail Forms, then a full moon came and the Jinchurikis' Chakra Tails appears and they started to transform into their Full Tailed Forms, and with the Jinchurikis' unlimited strength and power, they finish off the Nachurikis, and save the surviving prisoners. Even in death, the Narchurikis and their Negative Chakra Demon known as The Tailed Demon Immortals vows to be awakened in 3000 years into the future, to get their revenge on the Jinchurikis for destroying them.
History (Ninja World)
A Nachuriki's Zanpakuto is the same as it's shape, but with a new spirit within, the Zanpakuto's abilities and techniques are also transform into darkness along with the same techniques.
Shikai is a Zanpakuto's first form, along with the same power with it's wielder. The same with the second form known as Bankai.
Negakai (Opposite Release) is the negative second form of a zanpakuto, unlike regular Bankais, a Negakai's release is by summoning the Dark Zanpakuto Spirit by saying "When the shadows are transformed into even stronger shadow, a portal to the Netherworld opens and reveals a world without light".