Stories detailing the creation of the style of the mental grip are frequently historically impossible, and always at odds with each other on a number of key points, but in a few areas there tends to be agreement.
First, the style's creation has virtually nothing to do with Reshar, instead owing itself to a pair of mysterious figures known as the Dark Father and the Son. The Dark Father was a mighty psionic being of indeterminate race, whose face was never seen, always veiled behind a shadowy cloak, or black adamantine helm. But even given his great might, there was one greater still whom he served. Even less is known about this figure than the Dark Father himself, but historical artists frequently display him as an Emperor among mind flayers. The Dark Father resented his servitude to this Emperor, but found his power alone was not enough to break the bonds of his servitude. He pushed himself to his limit, and developed the rudiments of the Mental Grip, but could not fully exploit it; either he wasn't powerful enough, or the Emperor watched him too closely, or he was simply lazy, to drunk with his current power to try too hard to gain more.
Second, the Dark Father eventually found a student. In some versions he is the darker Son of the Dark Father, in others he was a master assassin the Dark Father sought out after an untarnished record of assassinating legendary psionic figures, rivaling even the Annulus in number of psionic beings killed. Regardless of his actual familial status, he is known as the Son. The Dark Father taught him all of his exotic techniques, including the bits of the Mental Grip he had initiated. The Son took these rudimentary fragments of a style and honed them to a razor edge; his killing power rose tenfold, and his power grew to eclipse even that of his Father. He wrote many treatises on the style, and it is from these that practitioners of the other styles eventually pieced the Mental Grip style back together.
Third, it was this mastery of the Mental Grip that eventually led to his downfall. Whether it was the Dark Father's jealousy, or the fact that his power grew too large to hide, eventually the Dark Father's plans were reversed; rather than allying himself with the Son against the Emperor, he was forced to ally with the Emperor against the Son. Though the Son's power was indeed great, he could not stand long against the combined might of the Dark Father and the Emperor.
What became of the Father and the Emperor is unknown - though there are a handful of cults that claim to have the truth - and largely irrelevant. The fact remains that the Mental Grip is a potent, if unorthodox style, and is growing in popularity among psionic followers of the Sublime Way. It focuses mostly on the use of small weapons, and those easy to manipulate with one's mind.
The way of the Mental Grip is made up entirely of supernatural techniques.