Kage no Honō

The Kage no Honō is an ancient spiritual energy passed down by the generations. It is currently wielded by Xallen.

History
The Kage no Honō is extremely ancient, dating as far back as feudal Japan. It was developed by an unknown person and passed down the generations in secret. Eventually, it was in the possession of a martial arts master who used it to defeat a younger Xallen, convincing him to come with him and train under him.

Xallen received the Kage no Honō when his master bestowed it upon him so that the raiders who have come for it would not speculate that the boy has it. Xallen, in a fit of rage, used the power of the Kage no Honō to annihilate the raiders' army. He then left for Sweden and trained in mastering the power of the shadow flame some more.

It is speculated that the Kage no Honō is actually a primordial sentient being who turned into a spiritual energy which ended up on the Earth, where Humans who were worthy enough were able to harness it's power. Another theory is that the Kage no Honō was a form of magic created by the Gods and fell to the lower world for mortals to harness. Either way, it has a mystical origin.

Usage
The Kage no Honō has to be mastered for it to reach it's full most potential. It grants the user powerful abilities such as energy manipulation and flight. It also enables the user to transform, raising their battle power. The Kage no Honō can also rarely talk to the user, giving them advice. It could also be considered a limitless power growth, as the user can keep getting stronger without limitations.

Trivia

 * The Kage no Honō is one of, if not the only technique that was not invented by it's current user.