Spoiler: On Developing the Cursed Seal
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To understand the the functioning and theory of the cursed seals, one must first understand their source. Their source is Nature Chakra, which is utilized in the legendary techniques of Senjutsu, only obtainable from becoming a sage of a summoning pact with an talking animal from another dimension such as Mount Myoboku or Ryuchi Cave to name only the most famous examples, but other pacts have done the same, such as the Shark Sage during the warring states era whose many dalliances has given people throughout the Land of Water shark features that have been passed on even today. The usual method of accessing Nature Chakra from what sources I can gather[1], is meditation to create stillness within yourself so that you can open up yourself to the nature energy around you. Essentially creating awareness through a void within yourself, silencing your own thoughts to hear the chakra of nature.
The results of accessing this Sage Mode are incredible. Everything is enhanced: strength, speed, stamina, reflexes, perception, durability, ninjutsu, genjutsu, taijutsu, access to new techniques using senjutsu, the ability to sense the chakra around you, it basically boosts everything. The only other such technique like it are the Eight Gates, a forbidden technique known only by Konohagakure, or in a more limited manner, Lightning nintaijutsu from Kumo. Why then doesn't everyone try to get this amazing ability? Because first, the dimensional animals are...erratic and picky at best about who they give their contracts to, not forthcoming about how their dimensions work, they are simply not cooperative and hold great power by themselves that we ninja have been unable to get around to get at their secrets. Even finding one of their dimensions is a great achievement, and the contracts currently known might be the more friendly dimensional animals: its speculative, but its entirely possible that many ninja have died discovering these lands unwilling to sign a contract or allow an outsider to know where they are and then killed them. If one got past all this, the technique itself apparently has a risk of extremes: too little energy drawn in, it doesn't work, too much and they turn into a stone animal of that dimension. One needs to moderate to find a balance of intake.
My goal of this project is to make a seal that will bypass all that and passively gather nature chakra to store for your own use, then when you tap into it, you transform into a hopefully similar state. I have named it a Cursed Seal for now as a matter of misdirection and working title, but have been toying with names like "transformation seal" or "Sage Seal" or "Nature Seal" so that it makes more sense for the final product. Though I do admit, the fact that it has a high chance of killing someone when used right now is a pretty big curse when you think about it.
Basically the entire concept is two parts: write the kanji "静寂" or "Stillness" in the center that will act as a symbolic representation of a person meditating in sage mode for the nature chakra to be drawn into. Then the second part is place various regulating patterns around it that will limit and regulate the amount of nature chakra drawn in so that it doesn't all rush in at once, without them the nature energy is basically sucked up like a vacuum and the user will be instantly turned to stone. Thus the goal is to design a set of regulating patterns to achieve the transformation without killing someone. At least most of the time, hopefully. Simple in concept, but in practice the flow of any chakra is chaotic and slippery, like a raging river or furious winds making it hard to completely control. Furthermore, sensor type ninjas have reported that even normal chakra can vary from person to person like fingerprints. Its just these little variances in flow and rhythm that make it hard to truly pin down.
Thus my seal has to not just find that ideal balance of regulation, but do so for as many people as possible. Its very possible due to various little biological variances that not all bodies will be able to work with the same kind of balance. Not every ninja has the same amount of chakra reserves, we definitely have different elemental affinities, finding a seal to be fully compatible with everyone is tricky because so much of it is tied up in medical information and anyone who works in that field knows that not every ingested medicine affects everyone the same way. There is always those edge cases that can't handle this or that. I'm hoping for 100% survival rate, but realistically I'm probably going to have settle for somewhere around 90% at an optimistic goal, but I won't go lower than 80% survival rate. The theoretical Mark One might be the 80% one and only offered on a purely voluntary basis with a liability contract to sign, because that 80% rate will only be in theory and more testing will have to be done on actual humans to really properly raise it and test to see if its true. The pie in the sky dream would be to design a seal that can detect your general chakra nature and adjust its flow regulation for your biology to raise the survival rate even further, but I'm not some Uzumaki seal master miracle maker.
Rats fortunately are similar to humans enough to use as test subjects. This project would never get approved by any shinobi nation if only humans could be used to test this out. Sanctity of life aside, it would be a great waste of human resources for no clear gain. Though some of my fellow researchers being ninja have....dark, morbid, one could even say macabre senses of humor and taste in decor and take the petrified rat failures home with them as decorations to put on their shelves to use as bookends, doorstops or just on their mantle. One of them says they are even collecting them to use in some disturbing art project all the rats are dressed up as ninja in some battle, complete with painted on blood. Its common joke around the lab that my real project is advertising these rats enough to start my own rat sculpture business, they ask every day "hey you doing more rat sculpting?" and I joke back "yes. I'll be sure to scare them so that they are petrified in certain poses so that I look like a mad scientist even more than usual." we laugh, its all good times.
This will take years yet to refine to the point of usefulness, but if I can pull it off, Otogakure will have an edge that will give it a chance against other villages without suicide tactics, and in my own way I'll have changed something, I will have contributed to something greater than myself and in doing so, proved that I can be a great ninja without my family's bloodline. That I don't need it to have an effect. What will a bloodline matter when potentially most if not every ninja in this village will be able to access a seal to fight against foes greater than themselves? When every ninja in this village will owe their strength to me, in a way I will outshine them.
To that end, I cannot allow my own sister to touch this work. I love her, but this is something I must do myself. If she contributes, they will just praise her for it. This is my work. My effort. It will be me who will be know for this. I won't stop her from acquiring it herself when complete- to have my own kind of legacy enshrined in Kimiko's quest for all jutsu by empowering her? There is no greater victory, for it will mean I will have a hand in her rise. No matter how small. No greater honor, for it will mean she will consider it useful enough to be something worth having early. Her own success in some way, will be because of me.
Because if my clan has taught me a single thing, its that true victory lies in the legacy you pass on to others.
[1]: One might be curious how I acquired such knowledge, considering the difficulty of even finding a contract holder, persuading them to let you sign, train for years to get to Sage Mode without being a sage myself. The answer is top secret, known only to myself and the Second Otokage Nozojo-sama, as I required his approval for this project.