'They say I'm a genius.'
'Everyone says I'm the next head of the clan after Dad.'
'They say there's a failure in the Zen'in clan.'
'Even though he's a man, he doesn't have a single millimeter of cursed energy.'
'I wonder what kind of pathetic person he is?'
'I wonder what kind of miserable look he has on his face?'
Zen'in Naoya would never forget the shock of the day he went to gawk at "Zen'in Toji," his heart filled with such childish arrogance.
A peerless body that had lost cursed energy—no, that had been liberated from it. Faced with an overwhelming "strongest" whose existence was on a different dimension, Naoya's body became convinced of its "defeat," and hallucinated the "death" that lay beyond it.
And what was both fortunate and unfortunate for Naoya was that, while he possessed more than enough talent for jujutsu, he was still a toddler who had not yet reached the awakening of his cursed energy.
As his body tried to resist death and force the cursed energy forcibly drawn from his dantian to surge throughout his body, "tremors caused by overwhelming terror" assailed Naoya.
However, since he had forcibly awakened to cursed energy at this critical moment, there was a slight delay in his action to cloak himself in it. Because of that, an impossible phenomenon assaulted Zen'in Naoya's entire body.
'Black Flash.'
'It is not a technique, but a phenomenon.'
'When cursed energy collides within 0.000001 seconds of a physical impact, space distorts, and cursed energy flashes black!'
Because the film of cursed energy he tried to wrap around his body collided with his own flesh, which was trembling so hard his teeth were chattering, Naoya was bathed in black lightning across his entire body by sheer coincidence. While burning the expression of Toji—who looked genuinely surprised at the "brat who suddenly self-destructed in the hallway"—into his memory, he lost consciousness from the severe pain of having his whole body and balls smashed by Black Flash.
Fortunately, since it was ultimately just the tremors of a brat, even with the power raised to the power of 2.5 by Black Flash, he got off with light full-body bruising. Also fortunate was that Toji remembered that the mysterious self-destructing shrimp in front of him was "Naobito's son," and that "the relationship between Toji and Naobito wasn't actually that bad."
Hoisted up easily by Toji, who intended to score some booze as a handling fee, Naoya was safely delivered to Naobito. Having his father find out about the episode where he smashed his whole body with Black Flash at the exact moment of his cursed energy awakening—a story where it was unclear if he was an idiot or amazing—Naoya would become the subject of drinking talk for a while after that.
However, what was unforgettably important to him was not his father's "affection," but the "core of strength" of the "strongest" man before his eyes, which he understood the moment he experienced Black Flash.
'The strongest existence known as Zen'in Toji is established by a powerful "Restriction."'
Young Naoya, having understood this fact from the bottom of his heart, would eventually impose "Ten Bindings" upon himself after consulting with his father.
This is the story of Zen'in Naoya in a timeline where he walked such a path of self-imposed binding.