The Light of Destruction, it wouldn't be exaggerating to say it's the true boss running through the entire GX series.
This cosmic antagonist first appeared in the second season of GX in the "Light Society" chapter, described as a cosmic evil with an endless impulse for destruction.
When Judai's trump monster, the "Elemental Hero Neos," made its appearance, the inhabitants of the new space explained to Judai about the eternal conflict between good and evil in the universe, stating that the Light of Destruction is the greatest enemy faced by the new space and the Neos.
And as everyone knows, the Elemental Hero Neos is actually just a civil servant from M78 in a new disguise. So the identity positioning corresponding to the "Light of Destruction" naturally becomes evident in the plot setting.
The Light of Destruction appearing in the "Light Society" chapter was the same force that many years ago possessed the "Blood Demon D" card. After Ed's father died, it transferred to DD's body and then eventually to the final boss of the chapter, Saiou Takuma, until it was vanquished by Judai and Neos's power at the season's end.
Moreover, the Light of Destruction tethered to "Blood Demon D" was already annihilated into nothingness six years ago by the "Slifer the Sky Dragon" commanded by Youyu, effectively nipping it in the bud before any real trouble could arise, so this whole chain of events shouldn't even occur according to logic.
But nothing is absolute.
For example, at this moment, even though Youyu had disposed of the evil influence residing in "Blood Demon D," the Light of Destruction still appeared, as if a joke of fate, once again choosing DD as its host.
Because back then, what Youyu defeated was merely a small fraction of the Light of Destruction's original body.
You can also see from the animation in the "Light Society" chapter that even though the Light of Destruction possessing Saiou Takuma indeed displayed considerable power, rising to a certain boss level, it wasn't quite at a world-destroying level.
That entity's plan to destroy the world at the time was by controlling the prince of a fictional country in the animation, seizing control of the country's satellite cannon, and attempting to destroy the planet through bombardment from Earth's orbit.
This portrayal shows that the entity's competence as a boss isn't all that impressive.
Dartz/Sok: A boss who needs human satellites to destroy the world, what a disgrace, just leave the group…
Yet, despite this, Youyu never became complacent thinking, "That's all the Light of Destruction is, I can take on ten of it."
After all, according to Judai's space companions' explanations, the real Light of Destruction is in the depths of the universe. Until the animation's conclusion, no one knew its true form or how powerful it truly was.
All that can be gleaned from the animation is that even Yubel, the most powerful boss throughout the GX series, only became mighty after receiving the Light of Destruction's waves in the universe.
The Light of Destruction's waves not only granted Yubel two forms she didn't have before but also gave her terrifying power.
The Yubel empowered by the Light of Destruction could suppress the Three Phantasms (albeit in card form), effortlessly break open the barriers between parallel dimensions, and with a snap, could teleport Duel Academy from the island to another dimension plane.
In that state, Yubel was truly unstoppable, defeating all the strong contenders in the GX animation. As an antagonist of Yu-Gi-Oh, Sister Yubel created the unparalleled "no defeat record" legend, and even in the chapter's end, the final battle with Yuki Judai ended without a clear winner.
(Legendary figures like Mr. Wang, Bakura, and Aigami all nodded in approval at this.)
And what's more terrifying is that the Light of Destruction appearing in this chapter is still just the tip of the iceberg of its true form.
In the chapter's conclusion, when Yubel was purified and merged with Judai, they also swore that they would fight side by side, jointly resisting the Light of Destruction which intends to destroy the universe.
However, the scriptwriters probably forgot about this little detail afterward, as twenty years have passed, and that plot hole remains unfilled...
But one thing is certain, and that is this entity cannot be underestimated. After all, the greatest ambition of the previous bosses has been to destroy the Earth, while this entity was created with the narrative to destroy the universe, which isn't far off.
And what concerns Youyu the most isn't even its power, but its mystery and unknown nature.
Among all the bosses that have appeared so far—no matter how powerful, like the Great Evil God Sock—you'd at least know its background.
The most terrifying things are really those unknown to people.
So here Youyu has already cursed the scriptwriters of the GX animation for leaving this plot hole unfilled, even wanting to drag the writers into this world to duel about it...
How to beat the boss can be figured out later, but first, Youyu must deal with the minions.
...And conveniently catch the Kaiju delivered to the doorstep by the opponent.
To be fair, a divine artifact like the Turtle Kaiju, capable of devouring Egyptian Gods raw, a weapon of ultimate destruction, has been offered with free shipping, how could he not accept it in good conscience as a duelist?
Youyu declares, after today, this turtle will be renamed Yuki!
I declare it!
"Duel!" x2
[Youyu, LP 4000]
[DD, LP 4000]
Although DD himself is trash, the Light of Destruction attached to him is not necessarily so. The Light of Destruction in the original storyline can elevate this heap of trash from a third-rate nobody to a world champion, and that's quite telling.
Youyu waved his hand decisively: "Set three cards on the field."
"Oh? Starting with three set cards, huh?" Someone observing the set back row not only failed to realize the gravity of the situation but even mocked, "It seems Mr. Wang's start isn't ideal.
Since the duel hasn't officially started, I can grant you a special mulligan, hehe..."
Youyu completely ignored him and grandly waved his hand—
—We're directly pairing with Reckless Greed!
Starting with three set cards plus a Reckless Greed leaves Youyu with only two cards in hand, netting four cards, while DD has five cards in hand, drawing just one.
The Duel King's draw advantage is simple, brutal, and quite bland.
Even the opponent is indifferent.
"Ha! You play a card draw enhancer right at the start, your hand must be terrible, huh?"
"Summon 'Magician's Rod'."
[Magician's Rod, Attack 1600]
A pale blue translucent magician's figure appeared, spinning a long staff in its hand.
"When 'Magician's Rod' is summoned, a 'Black Magic'-related Magic Trap Card is added to the hand from the deck," Youyu said, "I'll add 'Illusion Magic' to my hand.
Then Quick-Play Magic 'Illusion Magic' activates, sacrificing a Spellcaster-Type monster to add up to two 'Dark Magician' cards to my hand from the deck.
I'll sacrifice 'Magician's Rod' to add two 'Dark Magician' cards from my deck to my hand."
The magician who spun the staff vanished from the field, and two magician cards popped out of the deck, which he drew into his hand.
"Oh, you're using a Dark Magician?" DD scoffed, "Just a simple Spellcaster; in front of a powerful Kaiju, it's nothing but food."