Veteran Yu-Gi-Oh! viewers all know that although there have been seven series up to now, only DM, GX, and 5DS share the same universe.
Some say this was Mr. Takahashi's intention, as he personally participated in the planning of these three Yu-Gi-Oh! series and embedded a hidden thread throughout the stories spanning three eras.
In DM, the Great Evil God Sock mentioned the darkness in the human heart, and Sock himself is the embodiment of darkness, powered by humanity's own endless negative energy and dark thoughts, ultimately leading to the downfall of human civilization.
The ancient being Dartz also mentioned this point, stating that Orichalcos is a challenge to humanity, as people succumb to their inner darkness, leading to their own destruction a millennia ago, and Dartz prophesied this would happen again in the future.
Then in the GX era, the final antagonist Darkness in the anime conclusion similarly mentioned that humanity would one day perish due to their inner darkness. Darkness assimilated the consciousness of all humanity into an infinite cycle of illusions, claiming this method was humanity's salvation.
The various antagonists' prophecies and all the clues pointed to the same conclusion. And in the future revealed many years later in Yu-Gi-Oh! 5D's, these prophecies were fulfilled.
At the end of human history, Earth's civilization indeed crumbled due to the increasingly rampant darkness within future humans, turning Earth into a purgatory.
And standing before Youyu and others at this moment is a strange figure, the last human from the end of history, Z-ONE.
In a story set in an apocalyptic background, Z-ONE should have played the role of the savior protagonist, possessing extraordinary intellect, a compassionate heart, and the determination to save at any cost.
He had given up his own personality, body, even lost his original name in the river of time. He once harbored great dreams, was passionate and hopeful, sharing these freely with everyone around him.
He bravely stood up, bore many wounds, repeatedly rising from setbacks, supporting each other with his companions to move forward.
...But he failed.
He couldn't save anyone. He lost his family, friends, and everyone.
He became the last human remaining on Earth.
A failed savior.
Walking on shattered ground, standing at the desolate end of time, all that lies before him—the wreckage and corpses—are silent mockeries to the failed savior.
Dartz was right, and Darkness was right. Humanity is destined to follow the path of Atlantis. Generations of Duel Kings defeating formidable enemies and saving the world from crisis after crisis, ultimately failing to save themselves from humanity's own hands.
So Z-ONE returned from the end of time.
A savior who had exhausted everything but was powerless, eventually became the ultimate antagonist spanning the three Yu-Gi-Oh! series.
"...I guess you know Paradox," Youyu spoke, breaking the silence.
Z-ONE, seated in that steel apparatus, gradually started speaking. The voice emanating from beneath the metallic apparatus seemed mechanically processed, cold, and metallic as if it were a machine.
"Paradox mentioned you... I'm indeed surprised you managed to change his mind."
"It seems you two have communicated, and I bet it wasn't very smooth," Youyu said.
Z-ONE didn't answer that question, but instead said, "This venue has been locked by the 'Quantum Cube,' and no one can exit this dimensional field."
"...Give it back!" Aigami struggled to rise, "That belongs to us..."
However, Z-ONE ignored him.
"You know the rules, Duel King," he said, "One duel, one condition."
But Kaiba stepped forward, blocking Youyu, "I don't know who you are, but your action is extremely foolish! You must pass me first before challenging Youyu."
"No, Kaiba," Yugi also didn't back down, "This duel should be mine to handle..."
However, Z-ONE interrupted them, "I have no business with you, Muto Yugi, Kaiba Seto. At least not now."
The gaze beneath the steel gray mask went straight past the two, landing on Youyu.
"...Alright," Youyu thought briefly but did not refuse.
Kaiba narrowed his eyes, eventually humming lightly as he stepped back.
But as Youyu moved forward, Kaiba shot him a sidelong glance, arms crossed, "Just be careful. This guy...is no ordinary opponent."
Youyu didn't look back, "Hm, I know."
He had long anticipated this moment would come, just not so soon.
Youyu had been mentally prepared for this opponent in front of him since he defeated Paradox and sent the blond guy back to the future.
Z-ONE is not someone who easily gives up, nor a second-rate antagonist who turns over a new leaf with a few words of enlightenment. To make him believe that the future doesn't necessarily need to change through destruction or that hope remains, you must provide tangible evidence, not empty promises.
The device worn by Z-ONE opened components at the front, assembling a unique Duel Disc. The Duel Disc system activated, the Monster Zone was outlined by a golden line, and the electronic display for LP lit up.
But there was no Card Deck in the slot.
Youyu clearly remembered that in the final battle of 5DS, Z-ONE fought with Fudo Yusei using a deck built from large stone slabs. It was a deck named "Timelord," its power sourced from the "Kabbalah Tree of Life," a potent deck that was quite challenging in the anime.
"Your Card Deck..."
"As you know, I'm from a future time," Z-ONE said, "Due to some unresolved issues, my original Card Deck cannot be used in this time."
Youyu raised an eyebrow.
Unable to use it? Then what are you planning to use against me?
Playing cards?
"As a substitute, let me use this for now."
Z-ONE said calmly, raising his hand... or rather, the mechanical arm of the device on him.
The Quantum Cube suspended in mid-air suddenly seemed to be seized, stopping abruptly. Card after Card shot out from within the cube, lining up neatly as if a line of dancers entered the dance floor with precision.
They fell one by one at Z-ONE's empty Duel Disc, completing an automatic shuffle, and then automatically drew five cards from the top of the deck, floating before Z-ONE.
"My Card Deck!" Aigami exclaimed from the side.
"Alright, now I have a Card Deck," Z-ONE said calmly, "Let's begin."
Youyu: "..."
This guy...is actually planning to use the Cubic against me?
"Duel!" x2
[Youyu, LP 4000]
[Z-ONE, LP 4000]
"I'll go first, draw." This time, Youyu was going first.
The first card drawn was from Mr. Phoenix's gift. Youyu didn't hesitate much before using it.
"Magic Card 'Take Over 5.' Send the top five cards of the Card Deck to the graveyard." (Anime Card)
Five cards were sent to the graveyard in sequence, and Youyu then drew the next card from his hand.
"Summon 'Elemental Hero Liquidman'!"
[Elemental Hero Liquidman, Attack Power 1400]
"Liquidman's effect: when successfully summoned, Special Summon a Level 4 or lower Hero from the graveyard. I Special Summon 'Elemental Hero Blazeman.'
[Elemental Hero Blazeman, Defense Power 1800]
"Blazeman's effect, when summoned, adds a 'Fusion' Magic Card from the Card Deck to the hand. I add the Magic Card 'Fusion Substitute' to the hand."
Youyu revealed the Magic Card he had drawn earlier and inserted it directly into the Duel Disc.
"Magic Card 'Fusion Substitute,' Fusion of 'Elemental Hero Blazeman' on the field with the water attribute 'Elemental Hero Liquidman.'
The Fusion condition is any water attribute monster plus Elemental Hero. Fusion Summon!
Born from the abyss of extreme cold, freezing everything with absolute ice power—
——Elemental Hero Absolute Zero!"