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Part 1 of Eternity Enshrined
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Published:
2021-05-18
Completed:
2021-09-28
Words:
134,445
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15/15
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Eternity Enshrined

Summary:

Transported back in time, Megumi must rely on the only familiar face in Heian Japan. Ryoumen Sukuna, a curse to some and deity to others, decides to keep him.

The futility of Megumi’s plans distilled into desperation, which had him contemplating that name again: Ryoumen Sukuna, the only familiar one here.

But the people spoke of Sukuna in this era with utter terror. There was no air of triumph like in the modern day. Sukuna hadn’t been sealed yet, and the world knew him as the unbeatable, invincible King of Curses. He wasn’t the Sukuna that Megumi knew.

Canon-compliant through current manga arc and at least season 2 of the anime

Notes:

EE was completed on September 28, 2021, then updated with a re-revised version on May 30, 2022. The story itself was unchanged entirely, as I focused on line-edits for style consistency. If you want the AO3 downloads of the old version of EE (126121 words), here's a google drive folder.

This fic takes place in an intermission between season one and two of the anime. It draws heavily from canon and, while light on details post-S1, it is recommended for your reading experience to be familiar at least through Chapter 136 of the manga.

As of JJK Chapter 175, EE is mostly canon-compliant with the manga. The timing of events has been shifted to suit both the fic's timeline as well as the characters' power/growth: the Death Paintings/Yasohachi Bridge arc occurs at the end of the summer/early autumn in the manga, but here it occurs in mid-winter, and more time passes between the events of S1 and S2 of the anime.

Megumi is also aged up to ~20-21.

 

***
Huge thanks and love to my beta stolemyhheart who doesn't even read JJK. TY to cruellae for her sanity checks on Ch 1-5 and teasomnia for Ch 6. TY to shinigami_adam for the feedback on some chapters of the re-edit, and arachaion for feedback on JJK lore, accuracy, new content and more.

(See the end of the work for more notes.)

Chapter 1: mid-March to mid-April

Notes:

The beginning of this fic contains non-con of circumstance, by which I mean Megumi proposes an arrangement, but only under circumstances that leave him little choice. Sukuna accepts. It's unclear whether Megumi would be able to rescind consent safely, though he doesn't consider/try it. Sex is rough at times but not violent. After some time, their relationship develops, but there is still canon-typical violence and morality.

(See the end of the chapter for more notes.)

Chapter Text

I dreamed I was holding
A double-edged sword close to my body—
What does it foretell? It tells
That I shall meet you soon.

—Manyōshū IV: 604

The air pressure swelled as Megumi approached Sukuna’s shrine. Despite being amongst mountains, it felt like the world was underwater: the leaves, grass, rocks, and even light all hung heavily. Megumi had the distinct sense that he was descending to his doom, foreboding thick in his lungs. This was the last gambit of a desperate man. But waking up in a forest in Heian-period Japan would do that to a person.

Clothed in a dark kosode, Megumi's appearance betrayed nothing of his origin; only his phone, now scorched and dead, and school uniform, packed away in his bag, could prove that he had traveled through time. But the dread still tasted like bile in his throat even a full month after he’d first realized where he was—when.

The sun had been setting when Megumi first woke slumped against a tree trunk, his head throbbing. The terrible awareness that something was wrong had trickled into him like sharp ice over the next hour of wandering, during which Megumi found no path, no city, no Gojou-sensei or Kugisaki or Itadori. Instead he’d only come across an old-fashioned village with clothes hung out to dry, farming tools along the outer walls, and fires flickering for light.

He’d felt like a lost child, minuscule and alone and helpless with the huts and towering forests looming over him. Where was he? Hadn’t he just been fighting a curse with Gojou-sensei and Itadori? It’d been shaped like a crane, wings battered by the thicket, easy to suppress if not for how it had kept blinking in and out of his vision, freezing them in—in time. Could it be: similar to the tree and volcano-based cursed spirits, this crane manifested through fears of time and the lack of it, and Megumi had been struck by its wings, blacked out, and regained consciousness here, now, whenever now was.

Needing to speak to someone, truly anyone—for Megumi hadn’t been convinced he wasn’t dreaming, except he couldn’t rouse himself no matter what he tried, and Kon had seemed just as confused—Megumi skirted the edge of the village, trying to stay inconspicuous while looking in vain for any sign that his frightened suspicions might be wrong, but arrived at a shrine that only appeared to confirm them: olden architecture, but freshly built.

He’d knocked on the doorframe and, when the monk turned, bowed lowly. “Please, I need help,” Megumi had said, speaking as formally as he knew. “May I know where I am? And what year we are in?”

“Who are you?” the monk then asked, clearly looking over Megumi’s clothes and unkempt state. “A traveler?” 

After some thought: “A man seeking refuge.” 

The monk had frowned, the wrinkled, leathered lines of his face deepening. “And seeking more—the year?”

“Yes, the year.”

“Speak clearly,” the monk had demanded, gruff.

It’d struck Megumi then; although he was speaking traditionally, the terms he used weren’t known. “The season,” he’d tried. “The time period?”

Then at last, the monk understood. “You know not even of the child Emperor’s rule? It has been almost four seasons since the Later Emperor Ichigo ascended the Chrysanthemum Throne.”

Megumi hadn’t recognized the name. He figured he was likely in a time earlier than Meiji and earlier than Edo, too, since the monk failed to mention the shogunate. Distantly, despite the panic gnawing at his thoughts, Megumi had been grateful for his penchant for documentaries and even the recent wave of research assignments on ancient cursed spirits. He asked, tentatively, “Who is the ruling regent, while the Emperor is young?”

The monk stared at him suspiciously but gave Megumi an answer. “Fujiwara no Michinaga is the sesshou.”

The pieces had fallen together from that: the Fujiwara clan reached the height of their power in the early 1000s, which meant Megumi had been transported back a thousand years. He might have skipped out on history lessons in middle school, but Principal Yaga was stringent about his students learning jujutsu history, and the Heian Period was one of the highlights.

It was fortunate, in this unforgiving era; as the days passed, Megumi had known enough to not have to ask questions that would get him harassed, though he’d mainly kept quiet and to himself, trying not to blunder too obviously. The monk had been kind enough to let Megumi stay at the shrine for a week, at first to avoid the rain, then because it was apparent Megumi had nowhere to go, but Megumi did not venture further into the small town out of fear he might be provoke suspicion.

But it had worn on him to simply shelter in place, too. Passivity bred helplessness, and it compounded every moment that Megumi just sat there, eating bland meals of rice. From the moment he’d discovered he wasn’t in the modern era anymore, Megumi had been shadowed doggedly by distress, clinging to him like a curse. He’d been alone, horribly so, afraid to answer even the monk’s most inane questions lest his responses exposed his secret, and Megumi couldn’t even guess if divulging the truth would hurt or help.

He’d needed a solution, a way back, and surely, he’d thought, there must be something: a merciless era, yes, but also the Golden Age of Sorcery, that’d seen the rise of the three major clans, Sugawara no Michizane’s reign, and—Megumi had paused with a frown—the plague of Ryoumen Sukuna.

The name had rung in Megumi’s mind for weeks afterward, during the days with the curt but hospitable monk, who put him to work sweeping and washing clothes, and in the subsequent weeks, as well, when Megumi left to find the nearest jujutsu school to ask for aid. Heian-Kyo, former Kyoto, had been only a few days’ journey by foot, but the monk had been clearly reluctant to tell Megumi this, leaving him with more warnings than directions.

“Only death follows that path,” he’d said. Even when Megumi assured him of his convictions and training, the monk had sighed. “Even the strongest cannot win against a god.”

Megumi hadn’t understood then. But when he arrived at the capital city and found the jujutsu school, as his pulse quickened and world cracked and hope shattered, the name had echoed in his head again: Ryoumen Sukuna.

The sorcerers had been dead, nearly all of them—certainly all those who might have helped Megumi. A young sorcerer, stuttering in tears, recounted their demise: just one moon-cycle prior, the jujutsu sorcerers of the school had pooled their powers and attacked Sukuna only to be squashed like mites.

“His d-domain,” the boy cried. “I e-escaped, but I saw—and it felt—”

Devastating cursed energy had suffocated the school grounds, and no one powerful enough—no clan leaders, no Tengen—to help him had remained, but Megumi stayed regardless, due to some vague notion of duty, in case curses manifested from the despair. He’d assisted the cook, though portions were unfulfilling, and with minor menial tasks, mainly keeping an eye out and to himself.

On his fourth day at the school, Megumi had met a man with the family name Zen’in. Blinding hope had jolted through him: the Zen’ins would listen if he showed them his cursed technique. But just as quickly, that optimism had crumbled. The man wasn’t of the Zen’in clan; he simply was a Zen’in. Perhaps he was one of Megumi’s ancestors, but despite being a sorcerer, he owned no money nor influence. The stores of cursed tools and library of inexhaustible resources, the web of contacts and connections, the wealth—none of it existed yet. Megumi was a generation too early in time to be able to rely on the clan.

Pessimism had stuck like a burr, hooked into his skin. After a week, the downtime and forlorn air had spawned their own melancholy in Megumi; the ordeal was a never-ending, surreal nightmare he couldn’t startle from, and he had no plan, not even the vision of one.

He’d laid those nights on a hard reed mat, curled on his side, shoulder and spine aching, desperately imagining that he might wake and see Itadori there or Gojou-sensei, who would lean over him and say, “Wow, you sure slept for a long time, Megumi.” 

But the disconsolate mornings had come and gone, and Megumi had remained displaced a thousand years in the past, slowly desiccated of hope, as though it had been eroded from him by the watery rice, abrasive clothing, sleepless nights. After two weeks of the same—of nothing at all—the futility of Megumi's plans distilled into desperation, which had him contemplating that name again: Ryoumen Sukuna, the only familiar thing here.

But the people spoke of Sukuna in this era with utter terror. There was no air of triumph like in the modern day. Sukuna hadn’t been sealed yet, and the world knew him as the unbeatable, invincible King of Curses. He wasn’t the Sukuna that Megumi knew. 

Megumi had to have been crazy to consider it, or simply utterly lost, or perhaps both. All sorcerers were some degree of insane, yes, but Megumi had known his idea was well past risky and more foolish. The people remaining at the school certainly hadn’t understood, pleading with Megumi not to go when he’d asked where Sukuna resided.

Yet here Megumi was, a week’s journey later. He recalled the way Sukuna had been fascinated by him in the future, curious about his technique during their encounter at the detention center. Maybe, just maybe, he would deign to help him, if Megumi could keep his head long enough to ask.

At the very least, he had a better chance here than at the Imperial Court, which Megumi would have thoroughly failed to navigate, or finding Tengen, who could stay hidden indefinitely. Sukuna was the most powerful being in this era; only the three vengeful curses could graze his strength. If anyone would be able to bend time, it would be him.

Megumi knew Sukuna was unpredictable and quick to kill, not even only if he was displeased but simply on a whim. But from desperation bloomed stupid, half-baked plans. Was that not why Sukuna had first shown interest in him? Megumi’s bold scheme to defeat Sukuna into restoring Itadori’s heart—that had probably been just as unwise as his current endeavor.

Ever since their fight, Megumi had puzzled constantly over what potential Sukuna saw in him. He’d been toyed with, Sukuna with his hands in his pockets, commenting casually on Megumi’s combat, before the act had given way to a truer resentment, Megumi had felt, as the cursed energy whipped him just as meanly as the hits torpedoing him through buildings.

Yet even then—even then, Sukuna’s temper had quelled. Whatever anger he’d held at the world for his sealing, whatever he’d projected at Megumi simply for being present, he’d worked it out in every blow until finally calming and inquiring instead about Megumi’s technique.

Megumi didn’t understand it, but that strange interest was his one advantage now, the ace up his sleeve. He’d need all his cards on the table just to win an audience, after which point he could only bet his safety on intriguing Sukuna again, somehow. So that was the measly gamble.

His footsteps were loud and obvious, as ungainly as the plan. There was no birdsong in this place. Even the trees bent to Sukuna’s will; every dust particle cowered. Megumi felt the pressure in his knees. It clogged his diaphragm. Go back, go back, the shadows between bones and skulls whispered, along blades of grass, under Megumi’s feet. The battle could not even be fought, let alone won.

But Megumi wasn’t here to fight. It wasn’t boldness that propelled him onward through the wall of malicious energy, but fatalism and hope. He took one step, then another.

Sukuna’s shrine stood in a clearing within the forest, the entrance marked by a red torii towering over Megumi’s head. Further was the building itself, only a single hall with stairs leading to its doorway. The roof was ornate, and stone shaped as curved horns grew from it. From the slats—were those humans skulls? Megumi swallowed thickly and tasted death on the wind. The wild weeds rustled, gusts whistling through the animal skulls littering the grounds.

Sukuna’s cursed energy felt like an ocean upon his head. It was nearly impossible to move. I’m going to die, Megumi thought, knew. His instincts screeched. But he steeled himself, took an unsteady breath, and crossed under the archway.

A figure appeared at the doorway of the shrine almost immediately, shadowed by the roof’s shade. They had their own potent cursed energy, although small compared to Sukuna’s raging aura. “A jujutsu sorcerer,” they said. “You come to your death so willingly?”

“Not my death,” said Megumi. “A conversation.” 

The figure stepped forward into the light, revealing themselves to be dressed as a monk, with white chin-length hair. “There is no one in this world worthy to converse with Master Sukuna,” they snarled. 

Megumi brought his cursed energy to the forefront, readying himself just in case. “I have something Sukuna would like to hear.”

Rage contorted the monk’s face. “Addressing my lord by name? Show some respect, you scum!” they yelled. 

Their aura swelled, and suddenly shards of ice shot toward Megumi near the speed of bullets. Megumi ducked, rolled, and got his bearings in a crouch. One icicle flew past him and cold air gusted by. 

“I just need an audience!” Megumi shouted. “I have news from the future.” 

“Master Sukuna has no use for an oracle, especially not a pathetic one like you.” 

Cool mist channeled by the monk then surrounded Megumi, and he leapt out of the cloud right before a column of ice materialized where he’d stood. He only mostly successfully avoided being frozen; Megumi’s foot was caught. 

He brought his hands together and summoned Kon, who roared to the surface from below, breaking Megumi from the ice as he did. The monk wasn’t prepared for Kon’s speed; in moments, it covered the distance between them and crashed into Sukuna’s servant, sending them both flying back into the shrine, out of view. Megumi ran toward them. Then—

His blood shivered, and his heart stilled in his chest. He couldn’t hear over his fear. Malevolence itself stood in front of him, holding Kon in the air with a hand around his snout, muzzling him. His shikigami was struggling, lashing out, but Sukuna—this was Ryoumen Sukuna—was unmoved. 

Megumi released his technique before Sukuna could destroy Kon, and Sukuna glanced, unimpressed, at his empty hand, one of only four. He was different in this time: larger than Itadori by good measure, with doubled arms, and half his face obscured by some type of wood; more intimidating, more formidable, which Megumi hadn’t realized was possible; just more in every way.

Megumi couldn’t move; he didn’t dare. Sukuna would slice his head off, or the very air would do it for him. He tried to breathe and wondered if Sukuna would kill him for it, but after a few unsteady moments, Megumi gathered himself. It was too late to turn back. He had confronted Sukuna before, just like this, and in the same way, his hands had trembled then; but he had done it.

Megumi saw Sukuna open his mouth and expected it to hurt, somehow. But Sukuna said, with the same silvery voice, sounding bored, “Are all you jujutsu sorcerers this annoying?” 

Megumi was bowing before he knew it. “I am not a sorcerer from the school.” 

“A curse user?” Sukuna asked, but before Megumi could correct him, continued, “Well, whatever. I dislike disturbances.” 

He was next to Megumi in a flash. Megumi didn’t have a moment to even think of moving before Sukuna grabbed him by the throat, raising him off the ground like a doll. His grip was iron, and his nails stabbed into Megumi’s neck and drew blood. A helpless cry escaped Megumi before he choked as Sukuna squeezed, digging his claws in even further. 

Megumi fought against the pain and panic overtaking his senses. The only reason he wasn’t dead already was because Sukuna enjoyed playing with his food. But he’d been at Sukuna’s mercy the moment he stepped close enough to feel his aura, and he’d still come this far.

“Nue,” he gasped, and his shikigami screeched as it flew in. 

Megumi fell unceremoniously when Sukuna dropped him to evade, and he coughed, staggering back up to his feet while rubbing his throat. His hand came away smeared with blood.

With a single swipe of an arm, Sukuna swatted Nue away like a fly, and his shikigami became an orange blur until Megumi released it. He wasn’t trying to fight. What he needed was distance, time—an opportunity.

He turned and ran, trying to put space between himself and Sukuna. But immediately, he was wrenched back, Sukuna hooking the back of his collar and flinging him to the right. Megumi caught himself, but his ankle twinged on the impact. 

Sukuna wasn’t in Megumi’s line of sight when he looked up. His cursed energy was a thick smog, and Megumi couldn’t track him with it. He looked around wildly—

Then a force struck Megumi in the back. Air slammed out of him, and he gasped and tasted blood as he flipped, only for it to happen again a moment later as his body collided into something solid and rough—a tree. Everything hurt. His insides felt pulverized.  

Megumi struggled to his knees, losing his balance around the tree roots. In his swimming vision, he saw Sukuna approaching, slowly and self-assured. This moment was Megumi’s only chance.

Prostrating himself was easy, in the end, despite the anxious flurry of his nerves. Megumi bent low, touching his forehead to the ground, and kept his arms outstretched in front of him, palms facing up. 

He heard Sukuna pause in front of him. A second ticked by. Megumi flinched when he felt Sukuna step on his fingers, but he didn’t draw them away. 

“Not so boring after all,” mused Sukuna. 

Megumi didn’t dare glance up. He grit his teeth and kept still. Sweat dripped from his hairline onto his cheek. 

“Let us speak, then, jujutsu sorcerer. Your technique is rare,” said Sukuna. “Worry not, I find myself in a good mood today. You, after all, did not bring a pack of idiots with you like the last one.”

Megumi’s eyes widened; that was similar to what Sukuna had said to him back at the jail when they’d first fought. Hesitantly, he peeked up and squinted through his greying vision to make out the features of Sukuna’s face, regarding him with curiosity. When Sukuna didn’t cut his neck for having the audacity to look at him—dramatic, flippant curse that he was—Megumi pushed himself up more, until he was still seated but no longer bowing prone. 

“I am from the future—a thousand years—four-thousand seasons from now,” he said, willing his voice to not waver. “I can be of use to you. I can offer you—”

“Ah,” interrupted Sukuna, “another oracle. What is it about you people, so stuck in your own delusion that it pushes you to include others in it?” 

“It’s not delusion,” Megumi insisted, “nor am I an oracle. I am from the future and have…objects from my time to prove it. And stories, and knowledge.” 

“Objects can be fabricated. I am all-powerful, not all-seeing. How would I know they are from the future? And stories, too. For example,” Sukuna remarked with a grin, “listen to my story of the future: there once was a jujutsu sorcerer with black hair who foolishly bothered the King of Curses, and when he offered nothing of value, his throat was slit and his heart pulled from his chest.” 

Megumi said nothing but couldn’t help his flinch. Sukuna was murderous by whimsy. How could Megumi hope to convince him? His ribs hurt, throbbing with each galloping beat of his heart, and with Sukuna’s aura around him, Megumi could nearly imagine claws plunging into him. He was plagued with the memory of Itadori. 

Sukuna crouched down in front of Megumi, casual and at ease. The eyes on the gnarled, wooden side of his face were vertical, and Megumi couldn’t pull his gaze away. It was like meeting Sukuna all over again, being caught by the sinister gaze.

“So,” Sukuna said, “go on then, tell me why I should spare you.”

And so it came down to this: the last gambit. “Myself,” Megumi said. “I can offer you myself. Whatever form that might take.”

“Yourself,” contemplated Sukuna. 

He grabbed Megumi’s jaw, and his nails dug into his cheeks, squeezing, but this time he didn’t break skin. Megumi’s lips puckered forcibly, but he tried to stay pliant, wincing only slightly as Sukuna tilted his face this way and that. 

Sukuna huffed a laugh. “What a mouth you have on you. So daring.” 

Megumi grit his teeth when Sukuna forced his thumb between his lips, but the pressure was insistent, and not even a second passed before he let up. Sukuna’s thumb breached deep, stroking over Megumi’s tongue and prodding at his gums and teeth. The violation squirmed in his core, but Megumi didn’t bite and only stared up at Sukuna with thinly veiled apprehension. 

Sukuna pressed down on Megumi’s tongue, forcing his jaw open further. Megumi felt like a horse, having its teeth checked for health before purchase, and his heart thumped so loudly he would bet Sukuna could hear it. Sukuna kept him like that for ages, Megumi tasting the mud and blood and cursed energy on the pad of Sukuna’s thumb.

A pointed nail dug into Megumi’s bottom lip as Sukuna began pulling away, and that, too, Megumi bore in uneasy silence before, at last released, Megumi wrenched himself back and wiped his mouth. He glared, trying to figure out Sukuna’s game. 

“Stories and objects from the future,” Sukuna said, then repeated, “and yourself.” 

Megumi twigged to that—that Sukuna was interested in Megumi’s offer of himself. In what way, he didn’t know: his technique, his knowledge, his…body? The taste of Sukuna’s skin lingered on his tongue. 

“You seem to hold yourself in high regard, to offer yourself to me and expect me to want you,” said Sukuna. 

Megumi chose his words wisely. “However you regard me—that’s my value. I am simply…hoping that you might see the benefits of hearing me out.” 

“And what benefit does this give you?” 

Megumi replied, “You are strong—the strongest. And so you’re the only one who could hear my story and understand it, do something with it.”  

Sukuna barked a raucous laugh. “Flattery?” he asked with a wide grin. “If you are going to beg for your life, I prefer you to be crying.” 

But Megumi instead peered at Sukuna, eye to eye through the blood dripping from his brow—a gamble again, but he remembered Sukuna enjoying his temerity in the future. “It’s not flattery,” Megumi responded. “It’s faith.” 

That brought pause. Sukuna considered him. “Your words are pretty, but you are far from the first jujutsu sorcerer to try making a pact with me, seeking power beyond his means. It always turns into the same story—always so fucking boring.”

Megumi swallowed thickly. His heart was in his throat. “I know you in my time. One thousand years from now, you, Sukuna, are there. You know me. And you didn’t find me boring then.” 

Sukuna paused, then his eyes narrowed. In a flurry of movement, Megumi was lifted and pinned against a nearby tree; bark dug into his back and shoulders as he flailed slightly, and his feet, dangling off the ground, kicked ineffectively until he blinked and gathered his bearings. Then he forcibly stilled and only grimaced, trying to avoid Sukuna’s sharp nails around his neck.

With a light touch, Sukuna traced along Megumi’s cheek, down his torso. He drew a line down Megumi’s side, and grasped his hip, holding him. Another hand—the third—brushed against his left thigh. Air stalled in Megumi’s chest. He couldn’t move; he couldn’t even look. 

Sukuna cupped Megumi’s crotch, feeling him, and Megumi held his breath, his stomach somersaulting from Sukuna’s touch and the nerves. He shifted, struggling, trying to shrink away, but Sukuna’s hand simply followed.

He leaned in close, and Megumi could feel his smile against his cheek. “Tell me, little sorcerer, did I ever fuck you in your future?” 

“No,” Megumi managed. 

“Pity,” said Sukuna. “I wonder why.” 

His fingers tightened, pricking Megumi’s skin and drawing a wince. But then a shivery sensation cascaded through Megumi’s body starting from his neck: Sukuna was healing him. The foggy ache in his head dissipated, and Megumi’s vision cleared. 

Sukuna commented, “I tend not to enjoy when my meals fall asleep on me.”

Right. Megumi swallowed and felt Sukuna’s palm against his throat. “Here?”

“So eager,” Sukuna leered. A humiliated tremble burned through Megumi. He was hyperaware of every point Sukuna was touching him. 

Then Sukuna wrenched Megumi from the tree and at breakneck speed transported them through the forest. When they came to a stop, they were at Sukuna’s shrine, tall and imposing. 

The world swirled around Megumi as adrenaline surged through him. This had happened so fast; he wasn’t—wait—

With a clatter, Sukuna slammed the door open and tossed Megumi onto the floor. He landed on his back and scrambled to sit up as he stared at Sukuna. Tension rose. Again, Sukuna crouched in front of Megumi. One of his hands cupped Megumi’s cheek, and Megumi flinched.

He was expecting pain. But instead, Sukuna asked, “What is your name?”

Megumi thought about giving him a fake one. Perhaps that would explain why Sukuna didn’t recognize him in the future, even though they’d now met here. But lying would not win him any favors. He answered honestly, “Fushiguro Megumi.” 

“Then, Fushiguro Megumi,” drawled Sukuna. He pressed Megumi to the floor, and all four of his eyes pinned Megumi down like chains. “Prove yourself.” 

Notes:

The Heian period lasted from 794 to 1185 AD, marked by Emperor Kammu moving the capital to Heian-Kyo, former Kyoto. This era is characterized by a decline of Chinese influence in Japan and the development of what we’ve come to recognize as “traditionally Japanese”—clothing such as kimonos, food, literature, art, and culture.

Pre-Renaissance, before BC and AD were utilized for year-keeping, people used the regnal years of monarchs or the pope ("nth year of the reign of King X”) when tracking events. Japan still uses regnal eras officially; they are currently in the Reiwa era, marked by Emperor Naruhito ascending the throne on May 1, 2019.

Moon cycles and seasons were used to track time easily by non-scholars. Days were split into 6 daytime units rather than 12 hours, and each month was basically a moon cycle. The full moon occurred at mid-month, and the new moon was the last day.

The Fujiwara clan was a family that, generation after generation during the Heian era, effectively ruled over Japan by dominating/controlling regent positions in the Imperial Court. Fujiwara no Michinaga expanded the family’s power to its height by assuming various powerful positions himself, having his daughters marry emperors, and eventually becoming Chancellor.

The Emperor Go-Ichigo was a child when he ascended, around age 7-8 in 1017 AD, and Michinaga was regent throughout his rule, including when the Emperor grew older. Sesshou is the title given to the regent while the emperor is still a child, and kanpaku is the title for the regent "advisor" when the emperor is an adult.

Sugawara no Michizane was a political figure until he was dishonored and demoted. When he died in 903 AD, Imperial buildings were struck by lightning and members of the Imperial family died, which made people believe Michizane had become a vengeful spirit. To appease him, they built the Kitano Tenmangu Shrine in his name, which eventually led to Michizane being deified in the Shinto religion as Tenjin, the patron god of academics and learning.

In JJK, Gojou notes that the Gojou clan and Okkotsu distantly descend from the Sugawara clan and Michizane specifically, who is one of the big three vengeful cursed spirits in the sorcery world.

It is also stated that Tengen was helping spread Buddhism during the Nara period (710-784 AD) as the basis for sorcery.