The term "Indestructible Vajra" had not existed at first.
In ancient times, people knew only of "Skin-and-Flesh Tempering," not "Indestructible Vajra." Martial arts had yet to be systematically developed, and cultivators mostly relied on instinct and the "I reckon this should work" approach to make themselves stronger. Over time, more and more people trained their bodies to this level, and the ability gradually became famous.
Later, a Sage organized the principles of Internal Art cultivation and spread them widely. At first, cultivating Internal Art produced less immediate results than training the body. But after five or six years, progress in physical training gradually slowed, while progress in Internal Art did not. After ten years or more, physical training slowed to an infuriating crawl. Months, even a year or two of hard work, might yield only the tiniest improvement.
By comparison, Internal Art, which allowed steady progress, clearly had the advantage.
Gradually, "start with physical training, advance with Internal Art" became the mainstream approach to martial arts.
That remained true to this day.
Apart from a handful of schools, almost no martial artists continued to work hard at physical training after reaching a certain level. Naturally, the state of "Skin-and-Flesh Tempering" became rarer and rarer.
Of those few schools that still trained the body relentlessly, the best known were the Ascetic Monks of the Buddhist School.
They cultivated through hardship bordering on self-torture, engraving the power of Buddhist teachings into their bodies. Gradually, their bodies changed color, taking on a brilliant metallic sheen like gold.
The Buddhist School called this transformation the "Vajra Body." Normally, it appeared only when Buddhist techniques were used. But the Vajra Body attained through ascetic training was permanent. Even after death, the corpse retained that sturdy form, filled with the power of Buddhist teachings.
Thus, the