Myst's grandfather, Graeme, was a scholar in the Technic League who had been researching a way to improve the usability of technological firearms by solving the problem of their limited ammunition. The solution he came upon was a bioelectric charge generated by the user's body and channeled into the firearm via a port in a cybernetic arm. For years he labored on the concept and after years of tinkering finally completed a working prototype.
But although Graeme had attempted to keep his research a secret, jealous competitors in the League took notice of his research and the materials streaming into his laboratory, and deduced the nature of his research (though not its form). Graeme came under pressure from the upper echelons of the League's hierarchy to reveal the full nature of his research and surrender it to the league for 'safekeeping', for whilst the mages of the League possessed a rare few methods of maintaining the charges in the technological weapons, they jealously resented the prospect of renewable power for those weapons being available through a device that might find its way into non-League hands. His laboratory was placed under surveillance and he knew that it was only a matter of time before they came knocking.
Reluctant to give up his life's work, Graeme sought a solution. In his desperation, he looked to his grandson, Myst, who had served as his apprentice and assistant, and there was no one who knew more about the arm after Graeme himself. The prototype arm had a flaw - the device drew its bioelectric charge directly from the user's heart, but without proper calibration and due to several inefficiencies in the circuitry, it could not be used extensively without endangering the user (grit limits how often it can be used). But reasoning that Myst's young heart would be better-able to cope with the strain, Graeme installed the arm on his grandson and cremated the original. When the league eventually forced their way into the laboratory, they did not think to look under the sleeve and glove of the young lad who looked in fear as his grandfather was interrogated, then brutalised and finally murdered. Myst himself was beaten to within an inch of his life and left for dead.
As Myst struggled to pull together what was left of his life after that traumatic experience, he found that the flaw in his grandfather's prototype arm was greater than previously thought, for he learned (rather painfully) that the arm also the potential to glitch and draw a massive charge from his heart, leading to a heart attack. Furthermore, due to the arm's direct link to his heart, there was no way to safely detach the arm without triggering the glitch and risking a fatal heart-attack. Whilst Myst was able to jury-rig a temporary fix for the problem with the materials left behind in Graeme's laboratory, most of the materials that he needs for a more permanent fix were taken by the League.
Thus did Myst find the life of an adventurer thrust upon him. After gathering what provisions he could from his grandfather's laboratory, he set off into the harsh Numerian wastelands, praying to whatever gods of fortune that might be listening that he would find the components needed to fix his arm before the next glitch claims his life.
