Natural Energy was a higher-level form of energy than Chakra. The Three Great Sage Lands had stood firm in the Shinobi World for a thousand years because they possessed secret methods for sensing, absorbing, and using Natural Energy—the Sage Jutsu Cultivation Method.
As a research fanatic, when Orochimaru first learned that the Shinobi World possessed an energy more advanced and mysterious than Chakra, he was naturally overjoyed and threw himself into its cultivation without hesitation.
However, Orochimaru was also an exceptionally cautious man. When he discovered that the Natural Energy cultivation method taught by Ryūchi Cave—the Sage Jutsu Cultivation Method—had issues, he relied on tremendous willpower to resist the temptation of cultivating such an ultimate secret art and decisively abandoned Ryūchi Cave's Sage Jutsu.
But that did not mean Orochimaru had given up researching Natural Energy. Instead, he devoted himself to it even more frantically.
Natural Energy in the Shinobi World was far too sparse. Even in the sage lands, it was only somewhat denser than elsewhere in the Shinobi World, and still extremely thin.
With the technology Orochimaru possessed, he had no way to extract Natural Energy from the air. If he could not even extract it, how could he study it? Thus, his early research into Natural Energy had made virtually no progress.
That changed when Orochimaru encountered Jugo—a person born with the ability to absorb Natural Energy!
After meeting Jugo, Jugo's innate ability to absorb the free Natural Energy drifting through the air opened an entirely new door for Orochimaru's research.
By studying Jugo's flesh, blood, and bodily structure, Orochimaru created the Curse Mark, a secret method that allowed ordinary people to absorb Natural Energy as well!
Although this secret method—the Curse Mark—had many flaws, and only a very small number of people could withstand the backlash