Reien (霊炎; Literally: Soul Inflammation) is the name of an ancient technique recently rediscovered by Tokino Hanamitsu. Reien allows users to generate mysterious azure flames which embody the odd property of burning chakra and other spiritual sources of energy. Despite their appearance it has been state that this technique is not derived from the Fire Release, instead being composed of relatively "normal" chakra imbued with unique properties. As such Reien isn't either strong or weak against various elements, and only seeks to consume any chakra it latches onto in a brilliant blaze of blue. More particularly the flames generated with this technique only actively burn chakra that doesn't belong to their particular creator. Therefore, 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.
When utilized against human targets, or other creatures with chakra networks, the flames tend to burn out quickly after consuming chakra around where Reien makes contact. While the reaction between the Reien and the user's chakra can produce painful burns as a result, death as a result is likely out of the question. Lifeforms composed primarily of chakra, on the other hand, possess a constant source of fuel for Reien throughout their bodies. Thus, this technique was at time referred to as the Sacred Fire of Purification (精製の神火, Seisei no Shinka), due to its effectiveness against supernatural threats.