Reien

Reien (霊炎 Literally: Soul Inflammation) is the name of the mystical blue flames traditional wielded by Spirit Dragons that are summoned by select members of the Fuyutama Clan. While it appears much like normal fire, despite its coloration, it is not considered to be Fire Release techniques. As such the flames of Reien are unable to start fires and are not naturally weak against Water Release techniques. Furthermore, the type of combustion involved with these flames is unique. The flames witnessed in normal combustion are heat and light energy being emitted primarily from an exothermic reaction between hydrocarbons and oxygen. In Fire Release techniques the reaction is between chakra and oxygen, but the resulting release of energy is nearly identical. When performing a technique utilizing Reien the techniques use of the user's own chakra is limited to light emission, so that the "flames" are visible, and to shaping technique itself. Due to behavior of the flames they will not burn the user and will only produce heat in the presence of a chakra source different from the user. Despite this Seiryū, the leader of the Spirit Dragons, is able restrict his use of Reien from harming specific targets of his choosing even if they are engulfed in his flames, but this appears to be an ability unique to him.

While Reien is harmless when initially created by the user and only produces the form of blue fire, when it contacts a source of spiritual energy with a signature that differs from the conjurer it begins a powerful reaction. Utilizing the aforementioned spiritual energy as a source of fuel the flames of Reien will quickly begin to burn with an intense heat. Depending on the source and nature of the energy being combusted by Reien the effects felt upon the target will be different. Spiritual energy yet to be merged with physical energy in order to create chakra burns exceptionally quickly and usually cannot be felt by the target. As such, if Reien is utilized to a high enough degree to burn all of an opponent's spiritual energy, and thus killing an individual, the victim wouldn't feel any pain in the process. It should be noted that all cases, except when Reien is being used by Seiryū, it cannot kill a healthy human with a developed chakra network in a single strike. This is because the rate that the flames burn often cause Reien to snuff itself out before consuming a target's spiritual energy completely.

When it comes Reien's reaction when contacting different forms of chakra, Reien behaves differently depending on the complexity of the chakra's nature. Raw non-elemental chakra burns quickly, though not as quickly as spiritual energy and thus can cause minor burns due to the increased length of time that heat is released during the reaction. While such burns wouldn't usually be considered life threatening, Reien is able to inflict physical damage in this way. When contacting chakra that has undergone basic nature transformation Reien burns even slower increasing the potential damage due to released heat. Chakra from advanced elemental natures burns even slower and can cause even more damage in the process. The heat released from the burning of advanced natured chakra is quite dangerous and can easily leave life threatening burns. Due to the change in rate of how different forms of chakra burn barriers formed out of more complex forms of chakra can withstand Reien longer. It should be noted that while Reien can burn all forms of chakra, and just burns certain forms slower than other, the concussive force of an opponent's technique still has a chance to overpower Reien in order to deflect it.

When it comes to tainted chakra and demonic energies, the reaction to Reien is quite different from the aforementioned energy sources. While spiritual energy and the chakra explained above burn cleanly and leave no noticeable byproducts behind, tainted forms of energy act differently. While such sources of energy burn at a rate that rival that of spiritual energy, the combustion leaves behind a superheated ash-like substance behind in the process. It is because of this ash the event of being engulfed by this type of Reien combustion is excruciatingly painful for the victim. This form of Reien is so intense that it can even harm, though not kill, a tailed beast. This was observed when Seiryū was able to repress the One-Tailed Shukaku who had temporarily taken over Kami Fuyutama. It is because of Reien's exceptional effect against demonic lifeforms that it is usually referred to as a purifying flame. It should be noted that while Reien continues to burn continuously until it has consumed the energy that it has connected, if a victim can force the ignited energy from their body, they can prevent further harm. This emission process is the most difficult for demonic lifeforms, due to the rate of Reien's inflammation and the pain of the burning process.

While the Spirit Dragons only utilize emit these flames orally, Yokōkaze can conjure small fireballs in her hand to be hurled at opponents in addition to the traditional usage. It should be noted that the chakra used for these techniques is quite high leading Yokōkaze to utilize a 1:1 ratio of her chakra to ancestral chakra in order to maintain energy. As such Yokōkaze's Reien method requires roughly two thirds the amount of her own chakra that it would to perform a similarly sized and shaped Fire Release Technique, with greater sized techniques requiring greater amounts of chakra. Yokōkaze's skill with this technique has developed enough for her to create several Fire Release mimicry techniques using Reien's flames.