Ever since the ancient days, demons and fallen angels have been at each other's throats, fighting tooth and nail to rule the Underworld—what humans call "Hell."
The Underworld is split clean in two: the Demon side (the Demon Realm) and the Fallen Angel side (Fallen Heaven).
Demons make pacts with humans, collect their dues, and stack power that way.
Fallen angels, on the other hand, manipulate humans to wipe demons off the map. Then you've got the third group—angels—who follow God's orders and hate both demons and fallen angels equally.
This three-way war's been raging since forever, and it's still going strong today.
The Underworld's sky is a deep purple. No oceans, but there are some huge lakes. There's a whole zone that manages the souls of the dead. The place is about the same size as Earth, but the population's way smaller. Even if you add up all the demons, fallen angels, and every other race, it's nowhere near the human world's headcount. No oceans means a ton of open land.
No sun, no moon.
Time basically syncs up with the human world. The Underworld has its own clock, but so reincarnated demons and the ones living topside don't get totally screwed, the four Great Satans cast a special spell to keep everything matched up. That means no "I came back and a couple centuries passed" or "I spent centuries down here and only a few days went by up there." Same deal for Fallen Heaven.
The Underworld's split into nine major territories that make up the main living areas. You've got the Familiar Forest crawling with monsters, the Agares territory that's number one in farming, the floating island of Aglæs where they secretly make Evil Pieces, and so on.
A few hundred years ago, at twilight, something shattered the fragile peace.
The amethyst sky ripped open like paper, and a massive sword wrapped in world-shattering pressure came crashing down right into the heart of the Underworld.
It landed smack in the buffer zone between the Demon Realm and Fallen Heaven—a quiet valley that used to be covered in Underworld ferns.
The instant the blade touched ground, a ring-shaped shockwave blasted out from the sword. Everything within dozens of kilometers turned to charcoal in a heartbeat. Within a thousand-meter radius, the soil flashed metallic and hardened. In the blink of an eye, that lush valley became a barren wasteland where nothing grows.
That divine weapon from the heavens became the most dangerous bait in the entire Underworld.
Day and night it pulsed with seductive energy, luring every living thing in the Underworld closer.
Weak demons and low-rank fallen angels dropped dead the moment they stepped within a kilometer. Four-wing fallen angels and mid-class demons could barely get inside that range, but after three hours their bodies would fall apart. Even upper-class demons and six-wing fallen angels could approach the sword, but the second they touched the hilt they either exploded on the spot or turned into mindless puppets.
Over the centuries, the valley filled up with layer after layer of bones. Demon spines, black fallen-angel wings, even the occasional pure-white angel feather—all eroded by the energy until they crystallized into pale gray husks.
When the demons realized the valley was technically closer to their border, they immediately set up twelve layers of demonic barriers around the sword and claimed the whole wasteland as theirs.
That move was like tossing a match into a powder keg that had been building up for a thousand years. Fallen Heaven decided a divine artifact that didn't belong to the Underworld had no business being in demon hands, so they sent an elite squad led by an eight-wing fallen angel to take it back.
Seven straight days of blood and carnage turned the buffer zone into scorched earth.
Demons built walls out of corpses; fallen angels poisoned every water source with light magic.
Both sides claimed they were "protecting" or "researching" the sword, but nobody admitted the real reason—they all wanted that insane power that could erase even high-class beings in an instant.
Because whoever controlled this place could study it, dissect it, and eventually claim it for themselves.
And time? Demons and fallen angels have plenty of that.
In the end, two of the four Great Satans—Beelzebub and Asmodeus—joined the fight, and the fallen angels lost.
The Demon Realm officially annexed the valley and named it the Forbidden Valley.
For the next few hundred years, King Asmodeus led teams of demons trying to research and pull the "divine weapon." They scoured the Demon Realm for promising candidates to draw the sword. Every single attempt ended in failure.
Still, three of the four Great Satans (everyone except Sirzechs) were absolutely convinced: one day, a demon would pull that sword and become the demon god who'd rule the entire Underworld.
And now… that demon has arrived.
A cross-shaped rift tore open right in the middle of the Forbidden Valley. Ghost-blue light slammed against the metallic desert sand, throwing up blinding sparks.
The moment Gaio's boots touched the cold, glowing ground, the twelve-layer barrier screamed with the highest-level alarm the Demon Realm had never triggered in hundreds of years.
"Shut up already."
He flicked the guard of his knife with his left thumb. A single clear ding cut straight through the deafening siren.
The alarm died instantly. Gaio relaxed his furrowed brow and turned his eyes to the giant sword surrounded by piles of crystallized bones.
"Hah… hahaha!"
When he finally got a good look at the sword, he started laughing.
Half the blade was buried in the ground, but from what stuck out you could tell it was roughly 170–180 cm long.
(For reference: Dante's around 192 cm tall according to some hardcore Twitter calculations. Never found exact weapon measurements, so I'm going off the games.)
The center of the blade was pitch-black like cooled lava, covered in cracked patterns. Deep red light glowed faintly from the cracks like hellfire sealed inside.
Two sharp metallic claws extended backward from the guard—both hand protection and pure aggression.
At the base of the hilt was a single red gem—the exact same one that used to be embedded in the blade of Sparda.
"Devil Sword… Dante…"
Gaio whispered the name, and a huge grin spread across his face.
He started walking toward the sword buried in the ground. Suddenly, orange smoke materialized out of nowhere. A lazy voice drifted out of the rolling mist.
"Heyyy there, stranger. Dropping by uninvited—what's the big idea? That sword, maybe?"
"That sword's a treasure of the Demon Realm, y'know. You can't just take it. And all these crystallized bones on the ground… you blind or something? This is what happens to people who get too close without permission."
The lazy voice was clearly warning him.
Gaio stopped and glanced at the orange smoke. "A treasure of the Demon Realm?" He snorted. "In a way, yeah, this sword's got a lot of history with demons."
"Oh? Sounds like you know something." A light chuckle came from the smoke. "Allow me to introduce myself. I am Falbium Asmodeus, one of the four Great Satans of the Demon Realm. And you are…?"
"The Demon Realm I'm talking about isn't your Demon Realm."
Gaio ignored the question and kept talking.
"Huh?" Falbium was confused.
What the heck did that mean—was there another Demon Realm? In that split second of confusion, Gaio vanished.
The orange smoke exploded outward. Falbium's furious shout ripped through the air: "Space magic?!"
By the time he locked onto Gaio again, the guy was already standing right in front of Devil Sword Dante.
"Stop! You'll die!" Asmodeus roared.
For centuries, countless powerhouses had blown apart the instant they touched the hilt. There was no way this guy was an exception.
Gaio didn't even flinch. His right hand closed around the grip.
The moment his palm made contact—
BUZZ—!!
Crimson light surged wildly through the cracks in the blade. The sword shook like a sleeping beast finally waking up.
The red gem at the top of the hilt flashed blindingly.
The metallic ground cracked outward from the sword in a perfect circle. Rock that hadn't moved in centuries groaned. The crystallized bones around it shattered into dust under the red light.
Inside the smoke, Falbium's eyes went wide. "That's impossible… he's fine?! Could it be…!!"
Behind Gaio, a towering demonic silhouette made of blue flames appeared—its ferocious visage actually made a Great Satan shiver.
"You're a demon?! Our theory was right?!"
Falbium's shout echoed into the void with no reply.
Gaio felt a strange sensation in his fingertips. The sword that had been stuck in the Underworld for centuries was sending him signals through its vibrations.
The red gem at the end of the hilt flashed bright, then dim—like it was blinking. The metal claws on the guard relaxed their aggressive stance.
The burning heat in his palm suddenly turned playful. Crimson energy raced up his arm and shot back down, leaving a tingling itch behind.
This wasn't resistance. This was excitement. He could clearly feel the restless energy inside the sword—the same deadly, seductive waves that every creature in the Underworld found irresistible were now bouncing around his veins like an overexcited puppy.
Buzz—buzz—
The sword's vibration spiked. Every last crystallized bone in the area shattered into powder.
Gaio raised an eyebrow. Now he got it—why idiots had kept throwing their lives away for centuries. The sword had been actively tuning its frequency to whatever each victim desired most. Classic carrot-on-a-stick trick.
CLANG!
The blade rang out bright and clear, almost smug about its centuries of successful trolling.
Hundreds of meters away, Falbium's orange smoke churned violently. The divine weapon that had never let anyone touch it was now perfectly docile in Gaio's hand.
"Impossible… how is his energy compatibility this high…?" Falbium's voice actually shook.
Gaio ignored the freak-out.
He could feel the sword sending clearer images now: every greedy face, every trembling hand that had reached for it over the centuries, every expression of despair when they exploded. The energy carrying those memories was dripping with pure mischief—like the sword was proudly showing off its highlight reel of pranks.
Gaio smirked and flicked the hilt. "Treating killing demons and fallen angels like a game? Yeah, that's totally Dante's style."
"Time to come with me."
He twisted his wrist lightly. Devil Sword Dante reacted instantly—the half-buried blade jerked upward with a screech of tortured metal.
The hilt that hadn't budged in centuries lifted half an inch. The blade started glowing blindingly bright.
Falbium's smoke froze mid-air.
He watched the blue-flame demon silhouette behind Gaio merge with the sword's crimson light. Blue-red flames erupted from the cracks in the ground. The divine weapon that not even a Great Satan could move was showing absolute submission to a human.
Gaio didn't have time to enjoy the Satan's meltdown. The sword was practically begging him to pull. All that pent-up energy was dying to break free.
He tightened his grip. The blue glow of Yamato and the red glow of Dante's gem reflected off each other, pouring into his body. His originally brown-black eyes shifted—one blue, one red.
"Well then—" The corner of his mouth curved up. "Let's get out of here, Devil Sword Dante."
The moment the words left his mouth, the sword unleashed a deafening roar.
Red-and-blue devil flames shot into the sky, ripping a massive hole straight through the purple Underworld heavens, revealing a chaotic void beyond.
The metallic desert collapsed inward around the sword, forming a gigantic energy vortex. The sword that had slept for centuries finally awakened completely under its new master's will.
The instant the flames tore the sky, the laws of the Underworld let out a low, mournful wail. The air itself turned thick and black under the pressure.
In the Familiar Forest, hellhounds in the middle of tearing apart prey suddenly whimpered, tucked their tails, and bolted, spines bristling.
Flocks of bone crows smashed into cliffs in blind panic.
On the farms of Agares territory, the demon overseer dropped to his knees shaking, and every farmer behind him slammed their foreheads into the dirt, trembling like leaves.
Hidden four-wing angels stationed throughout the Underworld froze in place, holy wings twitching, their sacred light flickering.
Six-wing seraphim gripped their staves white-knuckled as the crystals on top cracked.
In the misty land of the dead, countless souls stopped drifting, curled into glowing cocoons, and prostrated themselves toward the flames, their broken prayers weaving into a chorus of despair.
In the floating island of Aglæs, Beelzebub looked up mid-meeting. His wine glass slipped from his fingers and froze solid in mid-air, red liquid crystallizing into frost.
"That energy…" His pupils shrank, his body turning transparent green.
Inside the obsidian palace of Fallen Heaven, Kokabiel—who'd just finished a blood sacrifice—tore open space itself. Blood droplets hung frozen around his armor. His eyes burned with greed and terror as he stared toward the pillar of flame.
Every single powerhouse in the Underworld turned their gaze to the same spot—the source of that red-and-blue column of light.
The Forbidden Valley, devourer of countless lives for centuries.
The moment the flames split the heavens, they all understood.
The divine sword not even the Great Satans could budge… was kneeling to its new master.