It has been mentioned before that the world of Yu-Gi-Oh! is actually filled with fantastical elements such as black technology, magic, aliens, and so on, a bit of everything peculiar and bizarre.
The anime depicts only the tip of the iceberg of this world. In the DM animation, the holders of several Millennium Items are the leading figures among the Dark Duelists, belonging to the top elite group of powers. But this definitely doesn't mean that Millennium Items are the only sources of supernatural power in this world.
For example, T Mister, whom Youyu encountered before, was hiding behind him, Darkness, a dark power system completely different from the Millennium Items.
Furthermore, in the Yu-Gi-Oh! GX Three Phantasms arc, there's a mysterious group called the "Seven Stars." Each member of this group comes from a different background of Dark Duelists, including elves, vampire descendants, alchemists, and so on.
The "Nightwalker World" that Youyu finds himself in now is precisely a place where such people gather.
To put it simply, this is a club for the Dark Duelists, somewhat akin to an Assassin Alliance.
The members here sometimes accept some commissions through the club as a platform, completing some grey tasks.
In the Darkness Game, the loser must obey the winner according to the established rules. A sufficiently high-level power user can completely turn the loser into his puppet. With such power, it's easy to force compliance from people in important positions or of important value, helping one achieve some special purposes effortlessly.
And if one wants to use the Darkness Game to erase someone, they can completely drag the person from body to soul into darkness, leaving no trace, making it impossible to investigate with conventional means.
Even if some Dark Duelists possess power like Darkness, they can even completely erase a person's existence through the Darkness Game, including memories of the person in the minds of family and friends, without anyone ever noticing that someone vanished from the world.
Many influential people in the world who know the secrets are actually aware of the existence of the Darkness Game. The organizations of Dark Duelists are not restricted by nationality or power; if anything, they maintain a near-neutral attitude, never deliberately favoring any side.
They will side with any country or individual offering a suitable price, maintaining good cooperative relations with many countries and powers.
It feels somewhat like mercenaries but is different in some ways.
Outsiders maintain a distant and respectful attitude towards the Dark Duelists because, possessing extraordinary power, Dark Duelists are seen as mysterious and formidable figures from the ordinary person's perspective.
However, during the GX era, there seemed to be some ordinary people pretending to be Dark Duelists using blinding techniques, hypnosis, and such to scam money.
Just like in the early GX series, when Titan uncle was hired to Duel Academy to duel with Yuki Judai, he pretended to be a Dark Duelist using things like light refraction and hypnosis but was ultimately exposed by Judai.
Youyu is currently in an era before the opening of the Duel City Arc in Yu-Gi-Oh! DM.
Before this, the undisputed king of the Underworld among Dark Duelists was none other than Pegasus J. Crawford, the founder of the card game "Duel Monsters."
Pegasus possessed the "Millennium Eyes," one of the Seven Great Millennium Items from Ancient Egypt, holding strong dark power and accompanied by the ability to perceive the minds of others, referred to as "Mind Scanning." Without a doubt, he was at the top tier of power users in the world.
Pegasus visited Egypt, obtained the Millennium Eyes, gained terrifyingly strong power, and under the guidance of the Millennium Eyes, converted the ancient Monster Tablets into cards, inventing the card game known as "Duel Monsters," which spread globally and made him one of the most renowned elites in the world.
As a game designer, he printed himself an unbeatable "Toon Deck," and paired with the mind-reading ability of the Millennium Eyes and strong dark power, Pegasus easily ascended to the throne of the king of the Underworld, becoming the unanimously recognized king among global Dark Duelists for a long time.
During that period, the Industrial Illusions Society controlled by Pegasus and its subsidiary hidden powers practically dominated the flow and momentum of the Underworld, a powerful force capable of influencing the world's arrangements from the shadows.
This arrangement lasted until Pegasus lost to a seemingly ordinary high school student, Muto Yugi, in his own Duelist Kingdom tournament.
Not only that, after that battle, someone even mysteriously took away his Millennium Eyes, causing his power to wane substantially, losing his former overwhelming dominance. Since then, Pegasus withdrew his forces from the Underworld brawls, living a life of retirement and seclusion.
For this reason, the ordinary high school student, Muto Yugi, has gained considerable prestige among the duelists in the Underworld (though Yugi himself is unaware).
Worldwide, Dark Duelists generally believe that if this boy named Muto Yugi were truly an ordinary person, he absolutely shouldn't have been able to defeat Pegasus.
Additionally, since Yugi possesses the Millennium Pendant, reputedly holding the supreme status among the Millennium Items, many believe that Yugi is now the most powerful Dark Duelist of the present age.
But quite a few believe the real strongest in the Underworld is someone else.
That would be the newly emerged group, Ghouls, which has been undermining many unwritten rules of the Underworld yet possesses indisputable strength... and their leader, Marik Ishtar.
Marik's clan, the Gravekeeper Race, has been a family that various powers have feared over the years, so much so that, if they wished, the Gravekeeper Race could potentially become the leader of the Underworld at any time.
The reason is simple; for years, the Millennium Items,
which hold the top-tier power in the Darkness Game, and the Duel Monster"
Realm's most powerful cards, the "Three Egyptian Gods," have been preserved within the Gravekeeper Race.
It's just that the Gravekeeper Race has historically adhered strictly to ancient traditions handed down from their ancestors, never abusing the power of the Millennium Items and Egyptian God Cards.
However, the current heir of the Gravekeeper Race, Marik, is apparently the first determined to break this rule. He unauthorisedly took the Millennium
Items and Egyptian God Cards, originally meant to be kept for the Pharaoh, and opened up irresistible conditions to recruit numerous Dark Duelists, forming the Rare Card Hunter organization "Ghouls," quickly
establishing a foothold in the Underworld.
Because Ghouls' operation style is rampant and barbaric, undermining many long-standing unwritten rules among Dark Duelists, there have been multiple
frictions between various organizations and Ghouls in just a matter of months, and many find Marik disagreeable.
Fortunately, Marik seems uninterested in material and status; his only goal is to march into Duel City and seek revenge on the Nameless Pharaoh inside
Muto Yugi.
Therefore, most powers have adopted a stand-offish attitude toward Ghouls, letting him and Muto Yugi fight it out. If Yugi can take down this lunatic, all
the better; in that scenario, Ghouls' forces would naturally crumble away.
But the "Nightwalker World," which Youyu is currently planning to join, is an exception.
Because, unlike others, the headquarters of "Nightwalker World" is located in Domino City.
The organizations of Dark Duelists have a strong regional consciousness; theoretically, this area should belong to them.
The head of the organization also has a fiery temper, stubbornly believing that "being invaded and rampaged by those Ghouls bandits on their own turf" is quite disgraceful. He explicitly stated that if the Ghouls dared to march their entire army over, they would immediately declare war in Domino City!
Were it not for fearing Marik himself might be genuinely formidable beyond his grasp, the head might have already led a team to Ghouls' main camp by now.
But thanks to the strong stance of the head of "Nightwalker World," Marik hasn't moved the entire Ghouls into Domino City.
After all, his target is simply Muto Yugi (or rather, the Pharaoh inside Yugi). Though he doesn't regard anyone other than Yugi, he prefers to avoid unnecessary complications if possible.
This is also why, in the Duel City Arc, Marik only selected a few relatively promising subordinates to participate.
And at this moment, what Youyu is preparing to do is related to the hidden mission of factional alignment—
—joining the Nightwalker World!