#tbhhhhhh

And this specific violin cover of the Last Goodbye on repeat:

(This headcanon came from discussions with @ketsueki-kujin)

Predating the existence of Ninshū, Onmyōdō was the precursor that focused on adherents of the Inner Path faith whom utilized natural energies in practices that allowed them to create many jutsu that came even before Indra Otsutsuki split from the faith his father created. One of the predominant tenants of Onmyōdō is that natural energy is able to be utilized in ways that excel past simply ninjutsu. As seen in objects such as the Stone of Gelel and in youkai that inhabit their plane composed of the thoughts and feelings of people that intermingle with natural energies and crystallize to manifest as sentient beings, even if many of them are perceived as things like demons. Originally, these were beings known as shikigami summoned by Onmyōdō practitioners, Onmyōji. Onmyōji were the priests within the Inner Path, the human Sages whom studied natural energies and were able to pass their abilities to their disciples.

One of the most prominent practices involved the conscious direction of thoughts and feelings into natural energies in order to make manifest items and other naturally occurring things--such as ores--that stores them that possessed specific uses. The earliest known precursor to Fūinjutsu, one of the best known examples was the Stone of Gelel despite being far from the only example. It’s said that the Slug Sages who managed to unlock refined Earth Release nature transformations could utilize this ability to store natural energies with specific powers and abilities that could be utilized to amazing and spellbinding effect. 

Such as with the ore the Stone of Gelel was harvested from--an example of a potent concentration of such manifestation--they can bear powers exalted enough to destroy half a continent, incur regenerative abilities on par with a jinchuuriki, cast illusions and blast sound waves, among many more. Being just one example of the Vein of Gelel, it’s in the Shikkotsu Forest that other exampes of these have been noted to exist.

Presently, the adherents of the Inner Path on Mori no Kuni are some of the only surviving, non-Sage practitioners who possess the ability that melds with their prowess over Earth and Water Releases. Though it’s unknown how it comes into being, it is believed that one must possess either Yin or Yang Releases, both, or Yin-Yang Release. With Katsuyu’s own mastery over Yin-Yang Release, she’s able to awaken or teach Onmyōdō in her students.

However, what makes In'yōdō (the complement to Onmyōdō) divorced from ordinary ninjutsu is the ability to create and summon spirits, such as the shikigami and other youkai that was abused somewhat by ROOT in the creation of Nue. It’s said that the specific summoning and harnessing of spiritual beings and their powers are achieved by In'yōji, which--while a pure form of Yin Release techniques unheard of in Nindo--are able to harness conscious beings and energies to utilize, such as what’s seen in Mōryō and Satori. 

Onmyōji, on the other hand, deal with Onmyōdō such as the Vein/Stone of Gelel that stores and creates powers for the user to utilize as per their creation. It’s said that the one who originally created the Yin Seals--such as what would eventually become the Byagokou--was a Onmyōji, Slug Sage, and later a disciple of Hagoromo who would record her techniques for posterity.