Name: All Life Submits
Type::Genjutsu/Funinjutsu
Rank: S

Abilities Gained:

Initially, Seishini was a victim of the technique known as Haunt, specific to a ninja named Ranryu. Ranryu was a high ranked ninja who was caught at his prime and wasn't ready to give up life. In his time, Ranryu served as a mentor-type to different families preparing them as a vessel to continue his life because he knew that his time had drawn finite. When found out about his devious plan he was hunted down and force to use his ultimate technique. At birth this technique was performed on the small infant (Seishin) and continued to exist with him. Invading his dreams on a nightly basis only served to bolster Saga’s knowledge of how the process worked. Ranryu had planned on taking over Saga's body when it was in a weakened state; that, or completely dead. The tides had turned on the mastermind though. All but dead after an assassination attempt, he tried to use the ability on Saga prematurely. This was a mistake, one not anticipated. Saga had the mental fortitude to and the sufficient viscera to, in effect, reverse the stasis of the Haunt ability on Ranryu, in the form of a technique now known as All Life Submits. Through the exploitation of the weaknesses—both in the jutsu and within Ranryu in his defeated state—he managed to lock the evil genius in a partition within his mind. Thus allowing the user to become the used.

This cohabitation has granted him complete knowledge to all of Ranryu’s secrets, from anything involving the Nisshoku to the Jekial, Jitijin, Fuuma, Uzumaki and Izanami. Anything involving his abducted bodies and the experiments concerning them, including mission history and recollected battle experiences. Nothing is hidden from Saga that Ranryu knew; nothing that he has experienced in his life. He may also draw on Ranryu and Kabuto’s congealed chakra reserves in direct correlation with All Life Submits technique. If he meets the proper conditions and level of skill, he may grow to learn techniques directly from Ranryu’s knowledge, but only if specialties and elements permit.
(This does not apply to bloodline-related things, even techniques. In fact, he doesn’t get anything like nor related to that.)

Disadvantages:

There are some things that are useless to him that Ranryu has, such as his high-level Genjutsu techniques that he will never be able to learn for example. This is one of the more unfortunate aspects, if anything. The real danger to these perks comes with overuse. Normally Ranryu is quite easy for Saga to control and force back due to his new compartmentalized development. But if he ventures into the area in his mind where Ranryu resides too often, without allowing himself a chance to mentally exhaust the negative influences inherent within, then he is liable to have his mind completely overtaken. This applies more if he is low (or out) of his own chakra and grows dependent. Worse still, there is also a chance that both of their minds will become lost; this would result in his body being a useless flesh heap.

One of my weaknesses is that I sometimes pay too much attention to details and while this skill set is crucial to projects with zero tolerance for errors, it can lead to analysis paralysis in cases of projects that require quick turnaround time.

Other Effects:

On the surface, there are no significant effects on his personality. It’s all but completely hidden from anyone that he has associations with. However, extenuating usage has caused him to become more violent in battle—though he still shows the ability to restrain them from taking over and predicting his actions. Despite this urge to utterly destroy those that he gets into squabbles with, this is nothing new, since he was born from a clan and an environment with abysmal moral virtue to begin with. He is now more perceptive of the environment around him, something that he had trouble with at times before his new found ability to focus on the minuscule.

Correct me if I'm wrong but, from what Im getting from this technique is that another shinobi is sealed within your body to the point to where its almost complete fusion.

Unless I'm misunderstanding the entire concept, to me, this doesn't quality as a Rare Ability at all, simply some person's Ninjutsu (Which is strikingly similar to Orochimaru's) used upon your character but to his advantage. It just seems like you're trying to combine the effects of Sasuke absorbing Orochimaru and Naruto being a Junchuuriki, which doesn't make too much sense to me.

Nevertheless, this would belong within the jutsu or seal category.