Spoilers ahead, read on at your own discretion.
Chakravartin, the only 'true' god known to have existed, was the one who gave Mantra and the ability to manipulate it to a portion of the human race. With that power, this subgroup of humans quickly advanced technologically, surpassing all the other sections of their race whom were left to live in a more "primitive" state without Mantra. Eventually, they reached their limit: the amount of Mantra they had would only take them so far. To his end, the advanced humans organized their society, consciously or not, in a strict and religious hierarchy. The lower class, the primitive humans, praised the noble class as gods (partly because these were the ones who could manipulate Mantra). Thus the nobles became known as Demigods, though whether they named themselves as such or not is unknown. Giving their prayers to the Demigods, the primitive humans became the only source of Mantra.
This rather peaceful existance would not last for long. Once again, the Shinkoku Trastrium - now the name of the Demigod civilization, the only unified nation in the entire globe - reached its limit: they needed more Mantra to further their civilization. A new method to gather Mantra was then born, the sacrifice of humans; effectively mass slaughter. By taking their souls and refining them into Mantra, the Demigods were once again on the path of advancement. However, this came with a cost no one expected.
Gaea, as the world was then referred to as, was enraged when the souls of mortals were not returning to it once they left their bodies, the cycle of life and death being grossly disrupted. In a fit of anger induced by Chakravartin, Gaea unleashed upon the world Gohma Vlitra in an attempt to take back the souls that had been converted into Mantra. The War of Creation, a perpetual cycle of Vlitra appearing and subsequently being barely staved off by the Demigods, had begun. Perhaps ironically, this war prompted the Demigods to build the Karma Fortress to destroy the catalyst of this periodic event once and for all.
Mantra, in its full form, is composed of eight "pure" mantras, those being Wrath, Pride, Greed, Lust, Vanity, Violence, Melancholy, and Sloth. In its natural form, it resembles vibrations similar to those emitted by sound waves and possesses very little power on its own. However, through its manipulation - be it either by the Priestess or by resonating it within the Mantra Reactor - Mantra can become an abundant source of power. When amplified like that, Mantra appears as a green mist or aura capable of powering machinery, healing wounds, and also capable of strenghtning those indulged in it to god-like heights.
Mantra also influences the speed in which a civilization advances technologically. For example, the Demigods' civilization, the only ones capable of fully manipulating it, have far surpassed the other human civilizations technologically, achieving spaceflight and near immortality through medicine and robotics before any of the others even thought about combustion and bacterias. Even so, one does not need to be able to manipulate Mantra to feel its effects. That is why each human city's technological advancement depends entirely on its location: Those in an area with a high concentration of Mantra will advance quickly, and vice-versa.
Mantra can be obtained through either prayer or sacrifices, the latter needing refinement (a soul into Mantra).
During the time that Ryu and Akuma entered Asura's universe (non-canon) it is quite possible that the two fighters have some how been affected by the Mantra. This is proven when both warriors have taken, and even survived, powerful attacks from Asura both in his six-armed Vajra form and his Mantra form. The Mantra increased their ki to that of the deities level and it seems to have increased the demonic ki of the Satsui no Hadou.
- Mantra plays a huge role in Buddhism, being described as the source of all life.
- "Pure" Mantras have different colors in their aura/mist form. Wrath is Red, Violence is Yellow, Vanity is Purple, Sloth is Green, Melancholy is Dark Blue, Lust is Light Blue, Greed is Orange-Yellow, and Pride is White.
- The Pride Mantra Affinity is considered the most special of the Eight because it is believed it is the one bestowed upon the being most worthy of ruling over Shinkoku.
- The Eight Guardian Generals individually utilize all the eight pure Mantras, and each draw his or her power from that specific mantra. Asura is the General of Wrath; Wyzen, the General of Violence; Augus, the General of Greed; Kalrow, the General of Sloth; Deus, the General of Pride; Olga, the General of Lust; Sergei, the General of Vanity; Yasha, the General of Melancholy.
- Five of the eight kinds of forms of Mantra are part of the Seven Deadly Sins.
- Of the other three, two of them: Acedia (Melancholy), & Vainglory (Vanity) are also two 'historic' sins of the seven deadly sins; the third, Violence is also talked of extensively in the context of the Seven Deadly Sins.