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There are many ways that demons may form, none of which should be cause for a joyous occasion. War, death, plague, all of these and more may cause the birth of a demon, a wispy soul rising from the depths of Hell and slowly gaining a solid form under the care of an older being. Generally, the more horrific and painful the event, the more powerful the demon will grow to be.
Despite this, the birth of a new demon child is always seen as a momentous occasion, prospective parents clamoring for a chance to adopt and raise the child for themselves. This obsession with children has led many demons to carefully observe the human world for signs of upcoming conflicts, hoping to nudge them in just the right way so as to ensure hardship.
When a massive plague began to spread rapidly across the human world, killing many of those who came in contact with it, many of the denezins of Hell watched anxiously to see what sort of child would form. It was almost guaranteed, with how bad the disease had become, and the young, fledgling demon would likely hold immense power.
Sure enough, on the first anniversary of the plague's outbreak, a small, dark wisp, power radiating off of its form, emerged in a flash of hellfire.
The news spread quickly, as soon enough the entire realm was holding its breath, waiting to see what demon would try and make first claim on the child. A youngling of such power would certainly be a desirable child of any high-ranking demon, and it was quite possible that another demon war could be started among the nobles over the rights to raise it.
Nobody expected the king to make the first move.
Hisashi Shigaraki, the Demon King, was extraordinarily powerful and completely childless. Most demons thought that he didn't need, or even want, a child. They could become a potential threat, after all, try to fight him for the throne. Many thought that there would never be another Demon Prince, that Hisashi and his brother, Yoichi, would be the last.
Nonetheless, not even a day after the child had appeared, Hisashi laid his claim.
Standing at the front of his palace, the man held the tiny wisp aloft, leathery white wings spread majestically behind him as he announced the new crown prince to the realm.
The crowd stood silently as the king asked the entire realm to challenge him for the rights to his child, not even the most foolhardy and reckless of demons daring to fight the strongest being in all the nine circles.
Satisfied, Hisashi turned his attention back to the wisp of a child, smiling softly at it as he announced the name of his son.
"Izuku Shigaraki," he declared, speaking both to the young demon in front of him, along with every sentient being under his command, his voice clear and confident. A name using the word for nine was the perfect one for a child destined to live and grow up in the ninth circle, in his opinion.
One by one, the realm's most powerful demons came to pay their respects and announce their loyalties to the new prince. Some did so immediately, gracefully, eager to meet their young ruler. Others came slower, eyes filled with jealousy, but still none dared to protest. All for One had claimed the child, after all, and none dared to question his decisions- not if they wanted to keep their power, their lives.
Soon enough, the introductions were over, having proceeded quite orderly, without any duels or fights- an unusual occurrence for demonic adoption, but sensible, giving the father. All for One smiled softly to himself as strode off, cradling the wisp of a child to his chest gently, humming an ancient lullaby under his breath.
There would be an extravagant celebration the next day to celebrate the occasion, but for now, he could hold his new child tight and dream of the bright future ahead.
He couldn't wait to see what the child would become.
Compared to humans, and even to many other mythological and magical beings, demonic children are incredibly slow growers, with childhoods that can last for centuries, even reaching into a millennia, if the demon is powerful enough. As such, even though Izuku only took a year to form from the strife of humans (an incredibly quick time-span, demonically speaking), it was over half a human lifetime later that he finally managed to assume a solid, proper demonic form.
This happened quite slowly, the wisps that made up his body slowly combining together under the careful and watchful eye of Hisashi. Gradually, he began to take on a small, humanoid form- legs, arms, a torso, and head. Sharp claws grew to tip the ends of tiny hands, while tiny, curling horns emerged from what would eventually become his hair. Wings unfurled, and a pointed tail coiled out behind him.
When all was said and done, years after the plague that had created him had long faded into human memory, Izuku's form finally completely solidified.
All for One was delighted as he took in the final results, the pleased gleam from his eyes matching the toxic glow of the child's in intensity. There was so much to teach the child- how to harness it's magic, how to command legions, how to manipulate humans for its own gains, so much time to learn and grow and change, all while under the care of Hisashi.
Izuku ended up being an avid learner. Practically as soon as he had properly solidified, he was chattering, asking endless questions about the world around him, especially about the various forms of demonic magic.
All for One was quite happy to teach his child, answering any questions he had about the realms and how they worked. This, he had expected to see from a child, from his child. He had displayed many of these same traits himself as a youngling, after all, and as such it was no surprise to see his young son adopting them as well. His magic was strong and powerful, and Izuku had quite the talent for adapting it to any given situation, at least as far as children went. This, too, was of no surprise to Hisashi. Every day it seemed like his son took after him more and more, in both appearance and action.
This made Izuku's other tendencies all the more surprising, to both All for One and the rest of the realm.
Hisashi had shown his ruthless nature at quite the young age, expertly manipulating others to get what he wanted, unafraid to use force if necessary. Izuku, on the other hand, was a much gentler soul, preferring to spend as much time as possible helping others and fulfilling his self-proclaimed ‘princely’ duties, oftentimes without thinking to tell his father where he was going or what he was doing.
Which led to All for One’s current situation.
“Izuku?” He called out down the halls, worried, “IZUKU? Where are you?”
Hisashi turned down the hallway and stumbled into the courtyard, stopping short at the sight.
Where his demonic army was training, or, well, should have been training was instead a disorderly circle of massive, terrifying soldiers all sitting peacefully around the crown prince, who was busy adorning them with lopsided crowns made of dead, wilting flowers. Beside them lay bowls of untouched soup (which, on closer inspection, were obviously left untouched due to the mud and several species of poisonous flowers that filled the liquid), as well as a large pot they had been 'cooked' and dished out from.
It only took a moment for the soldiers to notice the newcomer, jumping to their feet and saluting, sheepish expressions only intensified by the flowers adorning their heads. Izuku, too engrossed in his work to notice his father's approach, took a few moments more to react. But eventually, he looked up to pass his now completed crown to the next awaiting soldier, stopping short as he noticed who they were all looking at. A bright, blinding smile spread across his face.
"Dad!" He cried out, running forward to jump into his father's awaiting arms.
"Hello, little wisp," Hisashi replied cheerfully, ruffling Izuku's hair gently, "What were you out here with the troops today, hmm?"
Izuku beamed proudly, gesturing to his handiwork.
"I was making flower crowns, papa! So they could have something pretty to wear while training! And I also made them soup, so they won't get hungry and can train really hard!"
"Oh, really," the king replied, bemused, "Is that so."
"Yes!" Izuku responded, "It's like you said: a king is supposed to watch over their subjects, and since I'm the prince I should try and help them too!"
Hisashi smiled softly to himself. His son was quite the kind hearted individual, so very similar to his uncle, despite his origins. And though the troops were meant to be training hard for the next few hours, he supposed it wouldn't hurt to let Izuku spend some time with them (and it was best to avoid the troubles that arose when Izuku found other ways to 'help. The stable fire was still all fresh in his memory).
"All right, little one," he said, sitting down and nodding at the soldiers to finally relax their stance, "Would you like me to help?"
Izuku nodded brightly, reaching out to give another hug.
"Thank you Dad!"
The king of all demons, ruler of the ninth circle and beyond, returned the smile with a soft one of his own.
"Of course, my little prince."