It first made its debut during the first World Warrior Tourament, where Ryu was possessed by to defeat Sagat in a drastic effort. Orinally, Ryu scared Sagat with a single Metsu Shoryuken, it was later retconned in Street Fighter Alpha series that Ryu lost control of himself as well of himself by rising his fist up first, scarring Sagat with a Metsu Shoryuken instend. His attack took the the properties of Goutetsu's art, despite having never been taught it by his master Gouken, Although Sagat wanted revenge for his scarred chest, he never new that the scar came from the ki of the Dark Hadou that had took over Ryu until the events of Street Fighter Alpha 3.
The Dark Hadou is a form of ki that emanates greatly from oneself through the darker aspects of the natural human instincts, including the desire to survive, trample, and impose over opposition, and on a metaphysical level, the natural occurrence and manifestation of lose, decline, and destruction.
To tap into the Dark Hadou, a person must be so consumed with the desire for success and perfection, or have such an intense rage and hatred, that they are actually willing to destroy, as such, only people who push themseves to be the best at all costs can take full advantage of it, however, the untrained mind can be driven into a temporary insanity if one has not prepared for it. Learning the Dark Hadou is the first step in learning the most dangerous technique in their art, the Raging Demon.
Psysical and Mental Changes
For those that do succeed in controlling it, they become unbelievably strong, but at the cost of losing their entire humanity and compassion as a person, they will develop a cold, cynical detachment from themselves, as well as other people, and develop a murderous need and suicidal desire to destroy others. Once a person has taped into the Dark Hadou within them. It can be very hard to resist.
Goutetsu is the only person up to date that was able to use it's power without completely giving in to murderous and dark urges. When under the Dark Hadou's influence, a person's eyes will glow white or red, if one embraces it to a lower degree, their eyes will glow blue. If one has used it to their fullest extent for too long throughout years of their life, they will eventually loose complete control of themselves, and will eventually transcend into a fully demonic being, likely one similer to Oni.
Goutetsu was once seen meditating while tapping into the Dark Hadou, so he probaly encouraged or at least accepted it, but he did not lose his humanity like Akuma, this is likely because only used it to a degree. Which is why he did not show any kind of corrupting like that of Akuma and Ryu. This is also how his students Gouken and Akuma were in introduced to the power, Goutetsu would have had to teach the Dark Hadou to his students if he did teach them the Raging Demm, but Ryu (only in his evil state) can also use this move despite the fact that Gouken never did teach them the move, meaning that the move is an inherent quality of the dark power.
After Akuma fully gave in to the Dark Hadou, he lost all of his humanity as well as his compassion as a person, to the point of being a warrior who only seeks a fight that will cost him his life towards another warrior and a person who only wants nothing more but power, and took a demonic appearance to match his darkened soul. Goutetsu was actually pleased and somewhat impressed with his student Akuma for tapping into the Dark Hadou for a life or death battle for their school, or so he thought, Akuma wanted to tap in to the Dark Hadou fully to become more powerful and wanted to use it for killing those who he found worthy of killing insted of using it for enlightenment and good.
Goutetsu himself may have been unaware of hi student had lost all of his humanity and compassion as a person. However, the brother of Akuma, Gouken, tought a toned-down version of the art by removing the ki, as he truly hated it, since he believed it encounraged destroying after seeing how Akuma used it to take their master's life, he condemned his brother Akuma for corrupting his Chikara no Hadou and vowing to never teach it to anyone. Akuma may have tamed the Dark Hadou, but he could not stop the negative side effects of this dark power, although it is highly likely he had no desire to. His understanding of the Dark Hadou is so great, he can embrace it to a lower degree, giving lesser opponents a fighting chance. to prove their worth to him. However, he still cannot return to his human self, as he has used it to his fullest extent for far to long.
Oni is the result of Akuma finally becoming one with the Dark Hadou, as explained earlier, his apperance has changed to that of a larger, demonic being with power beyond that of Shin Akuma. This version of Akuma will destroy anybody and anything, rather then seeking out worthy opponents to do battle with.
During the events of the Street Fighter Alpha series, Ryu finally realized that the Dark Hadou was the culprit behined the aforemtioned victory over the honorable warrior, Sagat. Ryu tries to never let the negative aspects of the Dark Hadou consume him ever again, because he knows that he cannot control it, and does not want to destroy any of the warriors that he faces in battle who are strong by any means. Furthermore, even if he learns how to control it, he would still lose all of his compassion and all of his humanity as a person, as Evil Ryu's personality shows.
Although Ryu never fully becomes engulf by the Dark Hadou after the events of the Street Fighter Alpha series, several games present a hypothetical version of Ryu who has been consume by the dark power, to the point of becoming Evil Ryu. It was not until the end of the Street Fighter IV series that Ryu's foster father Gouken, was able to help him control its power by sealing off the negative aspects of it by using the Power of Nothingness, however, it is unknown just how long the sealing will be able to hold off his Dark Hadou. A demonic, more powerful version of Evil Ryu with an appearance similar to that of Akuma, appears in the Arcade Edition of Super Street Fighter IV, possibly to show how he would transform if the sealing of the negative aspects of the Dark Hadou was released or broken by another source of power.
Unlike most of the other incarnations of Evil Ryu that acts on their urges to destroy, this particuler version of Evil Ryu is very dark, brutal, harsh, uncompassionate and even immortal, going so far as to call himself a demon and calling his power pure evil. He throws away his all of his respectful ways as a warrior and a person, and his own humanity for power, he enjoys fighting the strong and truely hates wakness from all of humanity such as love, family, justice, friendship, and vengence, He also becomes very brutal towards people, such as women and children, and even that try to oppose him.