1991, early spring yet to arrive, Tokyo, Japan.
Deep beneath the city, in a vast underground cavern untouched by light, something stirred—an amorphous chaos of despair and desire, a mass of pitch-black filth.
Its existence alone was an abyss. The being that writhed in the dark was the very condensation of the world's malice.
"…"
The black sludge let out a sound that resembled a sigh, gazing up toward the high concrete ceiling of the sewers.
With that sigh, the entire dark sea beneath the city began to boil.
It was like the "black wave rising without wind" described in the Book of Revelation.
That black mud had a name—Kaelith.
He hadn't always been this way. Once, he was just an ordinary human.
Thanks to an excellent system, Kaelith had successfully crossed into the Type-Moon universe.
That was rare enough in itself.
Motivated by the spirit of a desperate last-minute student, Kaelith got himself a job right after transmigration.
His workplace: Tokyo's underground sewer system.
Who could've thought that even after reincarnation, he'd still be condemned to spend his life in a sewer?
But that wasn't even the worst part.
On his very first day of work, ignorant of the sufferings of the Tokyo populace, Kaelith was devoured by a mass of unknown black mud.
An early death and early reincarnation—so it seemed.
But even after being swallowed, Kaelith's soul did not perish. It struggled on, refusing to yield.
Perhaps because he was a soul from a higher dimension… or because his spirit naturally resonated with the black mud.
Whatever the reason, after six days and seven nights of struggle, the will of the black mud was extinguished.
Kaelith's consciousness had taken over.
At that moment, he came to understand exactly what he had become.
Throughout human history, this existence bore many names that inspired terror across the ages:
The Beast of Revelation.
The Number of the Beast.
Master Therion.
The Seven-Headed, Ten-Horned Antichrist.
The False Messiah.
The Beast of Sodom.
Satan.
The Great Red Dragon.
But within the Type-Moon world, there was one title that fit best—
The embodiment of desire and gluttony, the King of Blasphemy, the one who holds the Authority of Murder.
The Sixth Beast — Beast VI.
That said, right now Kaelith was only a larva—an unhatched cocoon.
He would have to collect seven Heroic Spirits through the Holy Grail War before his seven heads of sin could be born and his ten crowns of blasphemy awakened.
"…But I don't really want to destroy the world."
Kaelith sighed again.
He was so hungry.
Since becoming the Sixth Beast, an endless hunger gnawed at him every moment.
He had long since lost his sense of time.
A day? Three months? Ten years? Sixty-five million years?
Without proper senses, even the concept of time slipped away.
Then, one day—
Though no one uttered the words "Let there be light," a sudden radiance pierced the darkness.
"Huh? That's strange. I'm sure I placed a barrier to keep ordinary people out."
A clear, melodic voice woke Kaelith from his slumber. He lifted his faceless head and looked upward.
A girl stood there, glowing faintly amidst the blackness. She was small and slender, wearing a green dress.
Her pale-gold hair reflected her sky-blue eyes. Her doll-like face showed nothing but confusion.
Like a radiant flower blooming in filth—her name was Manaka Sajyou.
Through memories from both his past life and the "core of the demon god" within him, Kaelith understood everything.
This girl had stolen a Christian relic from the Church and buried it beneath Tokyo—to serve as the egg of the Beast.
Manaka Sajyou was one of the culprits who had turned him into a monster.
Yet, oddly enough, Kaelith didn't feel hatred toward her.
After all, this girl—more beautiful than any flower—was gazing upon a grotesque abomination like him with a gentle smile.
"You've been a good child, haven't you?" she said softly.
The fairy-like song echoed through the cavern, and even though Kaelith had no organs, he could feel his nonexistent heart pounding violently.
The fading will of the devoured black mud sent one final ripple outward—an overwhelming surge of inhuman desire.
It wanted to devour.
To violate.
To destroy.
To kill.
To defile.
To fall.
No—before all of that, there was something more primal.
The original, fundamental desire that preceded all others—
To be born.
Yes… I want to be born.
Then, the black mud made a sound—the very first sound every human being learns to make after birth.
A word that transcended nations, races, and colors—
"Mama."
The cavern without wind roared like a tsunami of darkness, releasing its newborn cry.
"...Impossible."
Manaka blinked in disbelief at the Beast who had already gained reason and language—then smiled.
"There are still three years before the Fuyuki Holy Grail War. You really can't wait to be born, can you?"
She smoothed her skirt and crouched by the edge of the black pit, resting her chin on one finger as she tilted her head.
"That's troublesome. To hatch, you'll need seven Heroic Spirits—or thousands of human souls as sacrifices. Can't you wait a little longer?"
Mama. Mama. Mama.
The black cocoon quivered.
I want to be born. I want to be born. I want to be born.
The formless sea of flesh boiled and screamed. Manaka, however, smiled serenely and reached out her hand toward the darkness.
"…Hehe. Such a willful child."
But the moment her pale fingers touched the mud, they began to dissolve.
"Eh?"
She looked curiously at her melting fingers and the bright red blood that followed—not in pain, but in wonder.
"Oh, I see."
Speaking half to the child and half to herself, the mother murmured softly—
"If a living sacrifice is needed… then isn't there one right here?"
"Mama… Mama…"
Realizing her intent, Kaelith convulsed in horror, his body rising in a desperate wave.
You must not cross this line.
Yet the girl who called herself his mother mistook the violent thrashing for joy at imminent birth.
She knelt down and spread her arms.
"Hehe… Can't wait anymore? Come now—eat me, my good child~"
Smiling, the girl embraced the mountainous black mud.
"...Ugh."
Her pained expression lasted only an instant before she was dissolved.
Her beautiful dress, her pale skin, her slender limbs, her lovely face—everything disintegrated the moment it touched the mud.
Even the Mother of Babylon, the so-called Empress of the Root, was as fragile as salt before the authority of the Beast.
The angel once told John:
The beast with seven heads and ten horns will seize the mother's treasure, strip her of her garments, devour her flesh, and burn her bones.
Thus, the prophecy of the Book of Revelation was fulfilled.
After the girl was devoured, the sea of flesh grew calm again, as if nothing had ever happened.
"…I see."
Then, in the silence, a boy's voice echoed above the dark sea.
"Though this human shell feels a bit restrictive… this sense of constraint isn't unpleasant."
And from the depths of the abyss, two figures slowly rose.
The newly born boy held Manaka in his arms as he stepped out of the dark tide.
The Beast VI, now in human form, had manifested.
Kaelith looked down at the girl in his embrace—she had opened her dazed eyes. The exhausted mother raised a trembling hand and touched the boy's face.
"…Kaelith, let's go home."