Seals were, in essence, jutsu in written form. A medium, usually blood or specially made ink, was used to carry chakra. That medium was physically twisted in all the right ways to force the chakra into the shapes necessary for a certain result. Of course, chakra had a spiritual and mental aspect which both complicated and simplified matters. Most importantly, it allowed complex movements to be replaced by simple symbols with meaning to the seal's writer.
Life energy was not chakra, and so making seals that manipulated life energy necessitated throwing away my predispositions and starting from scratch. First and foremost, life energy didn't twist into jutsu the way chakra did. Which was not to say it was useless, only that it had to be used in different ways. Secondly, ink didn't conduct life energy like it did chakra. Life energy traveled through things that were alive, or used to be. Blood worked, and was what I usually used, but skin, flesh, bone, wood, and vines all worked as well. Chakra and natural energy also conducted life energy, but you couldn't create structures out of pure chakra (unless you condensed it into crystal form, but that was far beyond my ability) and chakra-saturated substances were extremely inefficient. It was fitting that a power I'd gained from a god of pain and violence could only feasibly be used by hurting and killing other living things.
All this I'd learned through experimentation in the years since I stole a sliver of a god. Now, as the Elemental Nations prepared for war against Madara, I planned to put that knowledge to use. We needed every advantage we could get.
It had taken me two days of searching the forests around Konoha to find a clearing that worked for my purposes. It took another fifteen hours of taking measurements and scribbling diagrams and formulae furiously across three different notebooks before I pulled the large scroll off my back and began drawing out a seal.
I'd long ago learned how to include a chakra element in my seals, but only the basic five at first. Advanced elements, made by mixing two different basic elements, were more difficult to figure out. Most ninja couldn't produce advanced elements; if they tried to mix, for example, fire and earth chakras, the two would interfere with each other. At best, they'd cancel each other out; at worst… well that was considerably more violent. However, some people possessed a gene that allowed them to mix two specific elements constructively, rather than destructively, in order to produce a new one. But it wasn't until I had an opportunity to work alongside Tenzou for a while and study his chakra system while he used his jutsu that I finally understood how it worked. It only took a few days after that for me to make my first wood style seal. It was worth it, though, because watching Tenzou in action made me finally understand why wood style was such a big deal; it was the only thing I had ever seen that could create life energy from chakra. (I knew, in a scholarly sense, that sex created life, and thus life energy, but… ick. No thanks. I was pretty sure it worked differently, but I had no desire to study it closely to find out).
I was still working on separating the chakra-to-life-energy component of the chakra-to-living-wood of wood style, but for now I'd have to make do. It took about a half hour to create the seal, and another half hour to double- and triple-check it, but finally I was ready. I rolled up the scroll and stood. With one end set on the ground, the scroll still came up to my chest. I thought of the name I'd made for this, the name I'd worked into the seal, that described what it was for as much as what it would do, and readied my chakra.
"Sealing Technique: Wood Style: Forest Surrounding a Home."
The scroll sunk into the ground, ink blossoming from it and spreading beyond the trees. The earth shifted and rumbled and the trees moved and grew. The clearing shrunk and shifted until I stood in the center of a four-meter-diameter circle. Blades of grass flowed in the wind, moving almost imperceptibly until I was surrounded by Fibonacci spirals. Branches above and roots below stretched and connected, growing into a single, massive structure. Vines draped up and down tree trunks and along the ground, gathering an arm's length from the center. Moss bent and curved around trees, forming rings that stretched to the canopy like rungs on a ladder. Even the mycelia of nearby fungi were worked into the design, stretching along and between roots like a massive spiderweb.
A few months earlier, Tsunade had given me access to some of the Senju clan's writings on sealing, including some written by Mito-sama herself. It shook my views on sealing almost as much as that first conversation with Jiraiya had. Two things in particular stood out. The first was an entirely new way of looking at seal design. Seals usually took the shape of symbols and designs on a surface, but if all those figures were for was to guide and twist chakra into certain shapes, there was no reason it had to be two dimensional. If you used a sufficiently conductive solid substance, it was perfectly possible to create three-dimensional seals. For their constructions, the Uzumaki clan favored a specially-made clay that was easy to mold when wet and sturdy when dry. The Village Hidden by Whirlpools was as formidable and feared as it was not because it was covered in seals, but because its buildings, and even the layout of the village itself, were seals. The moment you stepped foot in their village, you entered their domain. And they'd had plenty of time to prepare.
I had used my first seal to precisely alter the landscape around me to create a second, three-dimensional seal that would channel life energy through the living organisms around me. Creating a seal that itself created another seal was possibly the most technically complicated thing I'd ever done, and the slightest imprecision could have drastic side effects, but it was necessary for what I wanted.
The second earth-shaking discovery that came from reading Mito-sama's notes was that I wasn't the first to experiment with life energy and seals. The Uzumaki clan had managed to glean enough knowledge of life energy to create a certain artifact that was powered by the life energy of whoever used it.
Standing in the newly-altered clearing, I stripped off my jacket and shirt, leaving only my bandages and dozens of seals. I might've been embarrassed if not for the fact that I knew no one was around for at least a mile. I sealed my clothes and supplies in hammerspace, and all that remained in the clearing was myself and a cage holding a small rabbit I'd caught earlier. There was a storage seal directly over my heart, directly over my Gate of Death, and from it I pulled a mask shaped like a demon. Shivering, I placed it on the bare dirt in the very center of the seal. That autumn air sure was brisk, I tried to convince myself.
There was one final step before the seal could be activated.
The cage around the rabbit was sealed away and the animal quickly grabbed before it could escape. It struggled briefly as I grasped its head and body, until a quick twist in opposite directions ended its suffering. Working quickly, I pulled out a kunai and carefully poured the creature's blood between the ends of the vines in a specific shape. A triangle, containing a circle and a single line, because symbols with meaning could replace complex movements and that symbol meant Death for me. When the rabbit finally bled dry I was done. All I needed was freshly spilled blood, the rest of it was useless and was placed in hammerspace.
Life energy was a limited resource; you were born with a certain amount of it, you used a little everyday through the process of living, and when you ran out you died. As such, I'd taken to measuring life energy in terms of seconds, minutes, hours, etc. of a person's life that it allowed for. (It wasn't the most precise measuring system, different actions and different people used life energy at different rates, but it was approximate and still a work in progress). I poured a few seconds of my own energy into the seal to activate it. The rabbit blood, still fresh with life energy, worked as a catalyst, cutting down how much I needed to use.
The instant the seal began to do its work I leapt back. The air above the mask shimmered and twisted. Space itself bent inwards and outwards, stretching and calling to another world. A figure appeared to answer the summons, clothed in white with a blade in its mouth and fires at its edge. The vines, previously settled on the ground, lashed out and wrapped around the figure. Their purpose wasn't to bind but to nurture, channeling life energy from the surrounding plants directly into the god of death, sustaining his presence for that much longer.
I expected to feel terror, but I didn't. The ice cold grip of fear was instead replaced by an overwhelming sense of weariness and the desire to simply lie down and not get up again. I ran chakra through my system to wake myself up and it partially worked, but the soul-deep weariness of a long life remained.
"Lost spirit." The Shinigami's mouth didn't move, nor did his voice travel through the air between us, and yet somehow I heard him anyway. Its voice struck straight to my soul and rose to my ears from there. "Have you come at last to rest?"
When I first came up with this plan, I hadn't been sure whether I'd need to use communication or violence to get what I wanted, but I was glad the Shinigami seemed inclined towards the former. "Half of the soul of a being named Kurama was sealed within you over a decade ago. I've called you here to request his release."
"A request? Most who summon me make demands and give orders. Few have bothered to simply ask."
"It seemed like the polite thing to do." I was making small talk with the god of death. When did this become my life?
"You've learned some manners since we last met, little godling." Wait, what? When had we met before? "I suppose I should respond in kind." The Shinigami's shape flickered and folded in and out, flashing impossible colors before coming to rest as a black robed skeleton carrying a scythe. "Is this more comfortable for you?"
On the one hand, no it wasn't. If I hadn't had far too much experience with the impossibilities of gods, my brain probably would have shut down from what I'd just witnessed. But on the other hand… kinda, yeah. Despite my years of life in Konoha, my mental image of death was still heavily influenced by the traditions of my first life. He (did the Shinigami have a gender? Was he technically an it?) actually looked like Death to me now, and that was a tiny bit easier for me to deal with.
Still, apparently good manners had gotten me this far, so I decided to keep it up. "How about you take whatever form you're most comfortable in?"
"Don't mind if I do," he said. "You're such a thoughtful host." His shape twirled once more, collapsing inwards and downwards until it settled as… a black dog? Not even a particularly monstrous dog, just a knee-high, vaguely wolfish mutt. The only thing even slightly off-putting about it was its glowing gold eyes.
I took a moment to process that. Then I took another moment, then one more just to be safe. I was uncomfortably used to the actions of gods attempting to break my brain, but they didn't usually do it through sheer absurdity. The preferred form of the being responsible for the end of everything was a puppy. Was it bad that I wanted to pet him?
I noticed the grass around the Shinigami was already dead, a patch of brown spreading out from the center. It was a firm reminder that I didn't have time to waste with further pleasantries or absurdities. Straight to the point, then. "I'm prepared to make a deal for Kurama's Yin half." I pulled out a scroll from my kunai pouch. "Part of one soul for part of another."
Mito-sama's sealing notes had been the final key in cracking Orochimaru's curse seal. With them, Jiraiya and I had managed to isolate the piece of Orochimaru's soul from the rest of the seal and remove it. Once it was gone, unraveling the rest of the seal became possible, but the removed soul had to go somewhere. We had designed a new seal to contain the pieces as we removed the cursed seal from Sasuke, Anko, and a number of prisoners from Sound. There were over a dozen in the scroll I held. Tsunade did not know I had taken the scroll, and she'd probably kill me when she found out, but that would have to wait until after the war. As long as we kept the world from ending, I'd take whatever punishment I was given.
"A soul for a soul is the traditional offer," the Shinigami confirmed. "Hmm. No thanks." He reached his back leg up to scratch behind his ear. "What use have I for soul pieces? All things die eventually, even the universe itself. Not even the White Snake is exempt from that."
This was not going how I'd hoped. "Orochimaru is a man who seeks immortality and has nearly gotten it. He spits in the face of death. You're the kami of death, doesn't that bother you?"
"Not really. I don't particularly care if he dies now or a millennium from now. Eventually he'll slip up and lose his life, and when he does I'll see his soul off to the Pure Land where he'll await reincarnation. Just like everyone else."
He didn't care? I'd assumed from his name that he was the spirit of death given shape, but if that were true then surely he'd be bothered when people didn't die when they were supposed to. It could be that he was just lazy, but that didn't fit what I knew of kami. If he truly didn't care, then maybe he was something else.
Then it hit me. "You're not just death. You're the cycle. Death, rebirth, and the travel between Pure and Impure Lands. You're the spirit of the whole process."
The Shinigami's ears pricked and his tongue flopped out of his mouth. "Very astute. Though it's more of a coil, really. There was a beginning, when the first light blazed forth in the sky, and there will be an end when the last star dies a quiet, cold death. I'm the only being that will see both ends of the coil, and all that concerns me is that it spirals on without interruption. Everything else is secondary at best."
I returned the scroll to my kunai pouch. "If you don't want a soul, what would be an acceptable trade?"
He leaned forward, face serious again. "I will reunite the two halves of the Fox, and I'll even throw in freeing the spirits of your former Hokages. But in return, you need to do something for me."
I gulped. What could such a powerful being need me to do that it couldn't do itself? When I asked as much, he replied, "the coil of life and death is meant to run in one direction. Clockwise, if you'll excuse the metaphor. Some of your chakra techniques that resurrect lives interfere, causing a soul to run counter-clockwise. Usually this sort of thing is nothing more than a mild nuisance, but the Edo Tensei is particularly problematic. I'll grant you a fraction of my power to aid you, but you need to use it to end the current summoner's plans and free the souls he's ensnared."
That was a far better deal than I could ever have hoped for. "The summoner is my enemy and I already planned on stopping him. You have a deal."
"Then it seems our interests align, Corpse Princess. Approach me and hold out your hand." Dead grass crunched beneath my feet, blades turned completely black and drained entirely of their life energy. Leaves in the canopy above shrunk and fell from branches, letting through beams of sunlight that slowly merged together, speckling the forest floor with patches of light and streaks of shadow. The draining only seemed to speed up as I stepped forward, until even the tree trunks appeared withered and weak. By the time I held out my hand, the vines that sustained the spirit's presence had shriveled to a hair's width. Moments before he disappeared, the Shinigami opened his mouth and dropped a single crystal stone the size of my finger into my palm. It was a swirl of equal parts black and white, mixing together to form the most intricate patterns.
"Alright," I said to no one as I stood alone in a dead patch of forest. "I'll add it to my collection."
x-x-x-x-x-x-x
When the war began, the shinobi had been divided up between five different specialized divisions. I'd been put in charge of the Third Division's sealing team, under Kakashi-sensei. It was my job to figure out how to stop any zombie summons we came across for good.
The very first thing I noticed about the Edo Tensei summons was that they had no life energy. Immediately, I threw out dozens of half-formed plans. I turned my attention from Jashin's Sliver to the newest stone that hung around my neck. I'd had little opportunity to experiment with it, and all attempts thus far had produced zero results, but the Shinigami had said it was to be used to help stop the Edo Tensei. If it was going to be useful, now would be the time for it to do something. I cycled chakra through Death's Boon and took a second look at the reanimated Seven Swordsmen of the Mist.
They looked wrong. The crystal let me see people's souls, I realized with a start. I hadn't noticed it before because most people's souls look like their bodies do and tend to take up the same space. But the souls before me looked twisted and partial. I could see where they stretched in impossible directions, tunneling through this world and into another one. I understood why the Shinigami didn't like this jutsu; it didn't just bring people from the afterlife into this life, it pulled them like taffy until they existed in two worlds at the same time.
I could also see how the soul was held in this world. A second soul existed in each of the summons, presumably from the person sacrificed to activate the jutsu, and the two souls were wrapped up and stitched together with miniscule threads of chakra. It was disgusting to look at, but it gave me an idea. If I could cut the threads that bound the souls together, then the summoned soul should snap back to the Pure Land like a rubber band.
"Kakashi-sensei," I said, using Gelel's Last Note to switch into my shadow state, "I think I have a plan." I shifted and stretched the shadows in my hand into the shape of a scythe. The shadow scythe was sharp and flexible enough to target the tiny threads, but more importantly, if I was going to play the role of Death's helper then I might as well look the part.