Mark of the hydra

A jutsu created to allow for a growing population of hydras(an infertile species) by converting humans into them in exchange for power.

It works similar to both sage mode and juinjutsu, by sealing a large amount of nature energy inside a human it will eventually turn into the species it has signed a contract with, in the case of users of this jutsu that animal species is hydra. However since just placing a large amount of nature energy into a human untrained in nature energy would just cause the human to release it all instantly this nature energy is then sealed with 4 seals, all of which, by default, are powered by the chakra of the human in question and a small portion of hydra venom.

The four seals have different effects when released

When no seals are released the nature energy and hydra venom mix is used to near instantly heal wounds. However as this process is costly in chakra, nature energy and hydra venom it is quickly released before too much of this mix is expended to heal wounds. This seal is called the seal of armor (yoroi no fuin)

The second seal can only be released after the first and requires the body of the user to be wounded allready. When this is released the users blood will spread over their body in a curse-mark like pattern consisting of curved lines and circles. This blood will spread over clothes without staining them. it grants increases in chakra reserves, jutsu intensity, speed and physical strength on a similar level as a perfect sage mode. this seal is called the seal of blood( Chi no fuin)

The third seal is called the seal of transformation( henkei no fuin), it can only be released after the second. It will force the user to undergo a wild metamorphosis into a hydra, a reptilian being with 4 limbs, possibly wings and 3 heads. usually they are capable of walking upright and possibly flying if they have grown wings, they start out with 3 heads and for every head chopped off they grow another. any other limbs are also regenerated near instantly while only 1 new one is grown. in this state they have the heightened chakra reserves, jutsu intensity, speed and physical strength on a similar scale as jinchuriki transformations.

after the third seal unlocks the users chakra used for the fourth seal( seal of the monster( seishin no fuin)) this seal is located on the humans brains and when released will degrade it into one similar to that of hydras, completing the transformation and forcing the newly created hydra to be expelled from human society and forced into their hydra clan. after the start of this transformation there is a 4 hour period in which the jutsu can be transferred or spread to another human being, strangly though this is not allways the case, as sometimes the jutsu is not passed on through physical contact and sometimes it simply spreads, spreading it does however reseal all seals.