Seigaio

The Seigaio is a dojutsu that 'evolved' from the sharingan over the course of several life times, developing unique abilities but also a steep price.

Origins
The Seigaio's evolution began during the first shinobi world war, before Madara and Hashirama made peace and formed the hidden leaf. A member of the Uchiha who had awakened the mangekyo sharingan had grown disenchanted with the war upon having to slay his own sister, which awakened his mangekyo. He left the clan and thus began to wander the earth in search of something to grant him peace.

He found peace in a female member of the Yamanaka clan who had been traveling in search of her own answers. The two had both abandoned their clans in search of answers and peace, and they found those answers in one another; and thus they began a family and so did the Seigaio's evolution.

Though the sharingan did not truly awaken in its descendants, its power seemed to remain ever present within them; over time the dojutsu changed and evolved, seeming to adopt some of the Yamanaka's powers into itself. This transformation continued and refined itself over generations. By the time that the second Hokage took power in the leaf the Seigaio had fully evolved, and the descendants crafted a name for themselves; the Heiumi clan.

Awakening the Seigaio
The Seigaio, similar to the sharingan, awakens in a state of intense emotion; in this case the Seigaio activates when the user performs an act of sacrificing themselves in the defense of something or someone that they care about. This triggers the Seigaio's awakening and it's ultimately ONE TIME power of erasing the damage done to the bearer of the Seigaio, rewriting it as an illusion. This power only activates once, never again being useable under any natural means for any bearer. Once awakened the Seigaio creates a black pattern on the eye, similar in the way that the mangekyo does, each pattern being unique from person to person.

Attributes
The Seigaio grants its user the power to sense chakra from all around themselves; they are also able to detect emotional states through a person's chakra. The Seigaio also grants the bearer the ability to see through genjutsu, however this of course depends on how much the bearer refines that ability. The Seigaio grants the user the ability to enter the minds of others and peer into their memories, or even...change them in order to sway or alter a person's nature.

However the most interesting and the most sought after power of the Seigaio is the ability to release their spirit from their body. This 'spirit form' is similar to that of the yamanaka's own mind transfer jutsu and the spirit transformation technique. This spirit form is able to enter the minds of others, effectively casting a genjutsu over their mind. From there they can either leave the person's mind trapped, allowing the genjutsu to wear off over a short period, or they can use that person's body as their own. But the truly coveted power is the spirit form's ability to tear away a person's chakra and mind from their body, effectively killing them in an instant while leaving no marks on the body. However the spirit form's chakra abilities are not limited to lethal force, but can also be used to transfer chakra to a comrade if needed.

However, in recent times, in a similar way as to how the sharingan was able to evolve into the mangekyo, a very small number of Seigaio users were born with another use of the spirit form; this evolution is the ability to send one's soul into the past and either manipulate themselves or another that is near the physical location; one indicator of someone bearing this power is also being born with the wind release chakra nature.

Weakness
The Seigaio comes with steep costs for its power; the powers of this dojutsu are extremely taxing on the mind and body as well as a drain on chakra reserves; extended use of the eye causes blurred vision and causes moulding chakra more and more difficult the longer it is active. If overused it will ultimately lead to severe exhaustion and temporary loss of ability to mould chakra as well as effectively making them blind. The only way to counteract the effects is for the bearer to rest and NOT activate the Seigaio until they are fully recovered.

However the Seigaio's steepest price comes from the unique chakra that flows through the Heiumi chakra network; even without the Seigaio's activation the Heiumi have a limited life span, the reason being that the very chakra that grants them their strength is also damaging to their chakra network; with the activation of the Seigaio this damage is increased and continues to increase as its powers are used. Ultimately this results in a catastrophic failure and rupture of the chakra network in the body, dispersing their chakra and killing the user from the shock of the sudden trauma. From the outside it appears as if the bearer simply stops moving for a brief few seconds before collapsing. It is impossible to revive them with medical ninjutsu or even most reincarnation technique due to the destruction of the body's chakra network. This catastrophic failure occurs between 34 and 38 years of age for a normal Heiumi, but for a Seigaio user death will occur between 26-34 years of age; if the Seigaio is heavily used, death can occur 1 or 2 years before the age of 26 depending on the degree of use.

Additionally it seems the chakra of the Seigaio can be altered by emotion. If a bearer of the Seigaio falls into the grip of intense hatred or malice it causes the corrosion of the chakra network to accelerate and if the Seigaio's powers are used by such an individual it will cause them pain as well mental strain right from the start; if they continue abuse the power of the eye then it will kill them, swiftly.