Hentai Shinken

Hentai Shinken(変態神拳; lit. Perverted God Fist) is an ancient, divine set of techniques in DxD: Twice Critical, created by the Major Shinto deity Izanagi and passed down to humans in order to counter Izanami's threat of killing a thousand people a day. A set of skills and techniques believed to be forgotten in time, it was actually resumed to only a single practitioner per generation in order to avoid overpopulation and the spread of heinous sexual crimes. It is the signature fighting skill of Hayato Kurosaki.

Summary
When Izanagi escaped Izanami from Yami-no-Kuni, the deceased Goddess, in rage, cursed that she would kill 1,000 residents of the living everyday before being sealed back in the Underworld by a boulder pushed by Izanagi. To retort such statement, he promised that he would give birth to 1,500 men everyday. However, after losing part of his powers by giving birth to the godly Trinity(Amaterasu, Tsukuyomi and Susano'o), he found himself unable to keep up against Izanami's curse even with his divine powers. Instead of giving up, Izanagi decided to lend his teachings to men, giving them techniques and skills that would allow them to charm, control and inpregnate a great number of women in a short amount of time, the set of skills being named Hentai Shinken, or the Divine Fist of the Pervert.

Despite his intentions,  practitioners would misuse such skills for evil cause and commit several atrocities with their learned abilties, causing overpopulation and subsequent hunger and shortage. Ashamed of his own naivety, yet refusing to give up on despair, Izanagi erased all the traits leading to Hentai Shinken but a single  practitioner, instructing him to take and teach a single apprentice, and to instruct him to do the same, making Hentai Shinken be regarded as a myth several generations later.

Abilities
Hentai Shinken is a martial art of divine origins which is said to be able to make one a 'Deity of Pervertness'(変態神; hentai-shin). Being restricted to only one master and apprentice per generation, making it almost impossible to copy or record it's history, it allows the user to control the Love(愛; ai) and Lust(欲望; Yokubō) into other's hearts and itself's  by hitting pressure points into one's body and unleash a myriad of effects that ranges from 'connecting two hearts for mating' to 'unparallel sex-drive', making it possible to charm and inpregnate any female, not restricted by race, years of strict training, tutelage and meditation allowing the practitioner to be able to control any ill urges he might feel with such power. Because of its history in secrecy, a few know ways to counter a Hentai Shinken strike.

Despite it's mainly use for repopulation, Hentai Shinken is also able to rejuvenate one's spirit, ease suffering and pain from women, heal wounds and broken body parts and more, also allowing the practitioner to defend himself against enemies and make ill-intentional people not be able to 'experience pleasure' ever again. Hentai Shinken is divided into two categories: The perverted sage's way(変態仙道; Hentai Sendō) which are the four basic techniques, and the Perverted God's'Secret Techniques'(変態神の奥義; Hentai-shin no ōgi), which are Four God-class secret skills that cannot be taught, but achieved when a practitioner reaches the apex of Hentai Shinken's teachings and ways. So far, only Hayato in 126th generations of practitioners has mastered all techniques.

Drawbacks
Hentai Shinkei cannot be used against those who have only Hate(嫌い; Kirai) in their hearts, since the base of such is around emotions such as love and lust. Also, Hentai Shinken has no long-ranged techniques, making the practitioner useless against foes which are far away. It can also only be taught from a human to human, meaning it has a relatively short time to learn and pass it down.

The Perverted Sage's Way Techniques
TBA

The Perverted God's Secret Techniques
TBA.

Trivia

 * Hentai shinken is a shout-out and a parody of the Hokuto Shinken fighting style from the Fist of the North Star series.
 * Despite the majority of its user's being male, Hayato claims that there were a few female practitioners as well. To teach a women such, however, is considered a tabboo for all the previous generations of teaching.
 * This is a set of techniques in order to counter Takumi's Narukami.