The Satsui no Hado, sometimes translated as the Dark Hado, is a dark energy-based force that is known for its use by Akuma.
The Satsui no Hado is a form of ki that is rooted in the darker aspects of the natural human instinct (including the desire to survive, trample, and impose over opposition), and on a metaphysical level, the natural occurrence and manifestation of loss, decline, and destruction.
To tap into the Satsui no Hado, a person must be so consumed with the desire for winning and perfection, be close to being killed, or else possess such intense rage and hatred, that they are willing to kill. As such, only people who push themselves to be the best at any cost can take full advantage of it, however, the untrained mind can be driven into insanity if they are not mentally ready for it.
Physical and Mental Changes
For the people who do succeed in controlling it, they become extremely powerful, but at the cost of losing their humanity and compassion; they develop a cold, negative outlook on life, a nihilistic detachment from themselves, as well as other people, and develop an obsessive, dark, murderous (and even suicidal), desire to fight.
The user also gains the ability to sense, and potentially influence, the emergence of the Satsui no Hado in others. Once a user has tapped into the Satsui no Hado within them, it can be very hard to resist in battle. Within the main series, Goutetsu is the only person known to date that was able to utilize its power without completely giving in to its murderous and darkest urges.