Zasshu Sennin Mōdo

ॐ is the sacred syllable that embodies the essence of all things. The syllable aun is composed of the three sounds a-u-n. The symbol's threefold nature is central to its meaning. Earth, Void, Heaven. In the Void, Earth and Heaven become one with the pulsating breath of absolute reality. A tune is struck that bears the note of the natural energy resonating throughout the whole world. Only the zasshu sennin (mongrel sage) may hear the holiest of sounds. Only the zasshu sennin may resonate to the most sacred syllable.

Mongrel Sages
A mongrel sage isn't one species of sage animal. They are twelve hybrid animals created by Rikudou Sennin at the time of his death and hidden around the world, the entirety of which is considered final sacred place. Their existence was not noted on the sennin's tablet or scrolls passed down to his kin. Only two such sages have blessed humans with their knowledge: the ware saber tooth and the dragon bird sages.

Overall Procedure
Training to enter mongrel sage mode requires one to "listen" with an 1) absent spirit, 2) empty body, and 3) open heart so that the sound of nature's breath may take up resonance therein.

Detailed Practice
In order to bear audience to this sound, the would be sage in possession of the required body must accomplish three complex tasks:
 * enter a state of mushin no shin, (無心の心), lit. the mind without mind, while gathering half the body's chakra reserves as yin chakra into the tenketsu in the upper (kami) tanden - located in the middle of the head between the eyes
 * while still in a state of mushin, release and hold the body's remaining reserves of chakra as yang chakra within the tenketsu in the lower (seika) tanden - located deep inside the 'hara'
 * in the single instant when the body dies, yielding perfect resonance with the sacred syllable, right before the gathered chakra dissipates and the soul leaves the body, flood the tenketsu in the middle (naka) tanden - located inside the chest at about the heart level - with both the gathered yin and yang chakras and simultaneously open only the eighth gate.

Execution
Normally, the gates must be opened in sequence or all at once. However, the last step can only be accomplished in this state of mushin (no mind) with no chakra flowing in the system, while the body dies to achieve perfect resonance with the breath of the world, and the dense harmonic chakra gathered within the naka tanden collects, through resonant macrosonic synthesis. a balanced quantity of natural energy. In this single instant, the eighth gate alone is forced open and the newly born sage chakra follows the path of least resistance by funneling into the chakra network, thus revitalizing the body.

Caveats
This process is fraught with danger:
 * If the acolyte fails to maintain mushin and his yin becomes unstable, he dies.
 * If he holds the second stage too long so his yang chakra dissipates, he dies.
 * If he fails to flow both his resonant chakras in tandem into his naka tanden or fails to force open his eighth gate alone, he dies.

State of Mind
The acolyte must breakdown the individual skills into manageable parts: achieving mushin, building yin and yang chakras, flowing chakras to the three tanden, opening gates. Some of these tasks can be practiced in combination until mastery is reached. Howver, since one cannot practice to open the eighth gate without perishing, the apprentice has exactly one chance to form the aun seal and achieve mongrel sage mode. When the acolyte has mastered all the individual skills and all but one of the gestalt skills, only then will he attempt to form the final seal.

Chūin: the Half-life
If he is successful, the perfect zasshu sennin enters a state of chūin (中陰), an intermediate existence between body's death and rebirth. The mark of the sacred syllable appears on his chest. He oscillates between life and this intermediate state in perfect harmony with nature, never quite dying. Essentially the true zasshu sennin remains in a kind of half-life exhibiting a form of permanent sage mode until the day he fully perishes.

Lifespan
Human mongrel sages maintain this state until the point in maturity where their resting chakra capacity falls below the threshold for drawing on nature's energy. This shortens the zasshu sennin's life span considerably. Hence the saying, a mongrel sage is not an old sage. To put another way, the bulb that burns far more brightly lasts not nearly as long. Depending on their initial capacity and maintenance of proper health, humans can resonate with the sacred syllable well into their 50s and a few even longer. However, beyond that point their remaining lifespan is measured in heart beats, rather than years.

Combat
The zasshu sennin also must be careful not to allow his chakra to drop in combat below the resonant threshold or death follows swiftly. That threshold stands firm at 20% of his total chakra capacity. Therefore the sage art of tsuiwonasu or twin (対を成す, lit. to act as an opposite;  to form a counterpart;  act as a twin) must be used to alleviate the necessity of resting during combat to replenish chakra. A tsuiwonasu is essentially a sage chakra clone, similar to the chakra clones used by the 4th Hokage. Upon mastering the sage arts, the sage seals the tsuiwonasu within himself. The sage then replenishes his spent chakra with rest, as does his twin. Once rested a mongrel sage possesses twice his normal chakra capacity, considering his twin. The twin remains sequestered in a special place within the zasshu sennin's seal, separated from his normal chakra. In combat, as the sage's chakra drops towards the lower limit, his twin replenishes his chakra and then rests within him to rejuvenate itself. If the sage uses his chakra faster than the rate at which his twin can revitalize him then his chakra falls below the level of resonance, spelling imminent death.

Genjutsu
As an added benefit the tsuiwonasu renders the sage immune to most forms of genjutsu in a manner similar to jinchuuriki. In fact, it is superior to jinchuuriki immunity because in the event of being targeted with genjutsu the twin can detect the irregularities in the sage's chakra network, sieze control of the body to maintain movement, and break the sage out of trance before returning control.

Sustenance
A mongrel sage does not require food, water, or air. Though he may partake if he chooses, he relies on sage chakra alone to replenish his bodily needs.

Offspring

 * a sage may not have children by a mate, who is not a sage herself. Doing so results in death to the mother and child.
 * children conceived after sage inception are born in this resonant state, have increased stamina, and therefore exceptionally long lives for mongrel sages, though, most likely still reduced by comparison to normal people. Such children may even survive until seventy.

Advantages
The user('s):
 * has all five senses boosted significantly.
 * only has to rest to replenish his sage mode.
 * gains shôkyô shin "the mind that plays across objects"
 * develops the ability to regenerate injuries, similar to byakugo.
 * feels no pain, recovering far more quickly from physical damage/fatigue
 * can harness the ambient natural energy, as an extension of their body, increasing striking distance.
 * physical strength, speed, stamina, reflexes, and durability dramatically increase.
 * ninjutsu, genjutsu, and taijutsu become far more powerful.
 * capable of projecting natural energy in order to temporarily breath life into inanimate objects or substances.

Disadvantages
The user:
 * cannot drop below the threshold of 20% of total chakra present in the body's pathway system without his chakra falling out of resonance with the sacred syllable, resulting in instant death. This is about 10% of total chakra when considering his twin's doubling of chakra capacity, at which point retreat and rest or death is imminent.
 * shortens life span by around one to three decades depending on initial chakra levels and healthy lifestyle.
 * upon mortal injury over more than 50% of the body or decapitation, chakra depletion past the twin's ability to replenish occurs, resulting in immediate death.
 * knows there are no imperfect mongrel sages, only acolytes, perfects sages, and corpses.