As The God of Darkness in Another World

Well, this novel is both fanfic and an original novel

The main character is in a mixed world

Well, it's a bit confusing, so let me explain with an example

In the fantasy world to which the main character migrates, there are four true dragons from "That Time I Got Reincarnated as a Slime."

But Rimuru does not exist

There are also seven primordial demons, but as I said, Rimuro doesn't exist.

On the other hand, there are also octagram demon kings and one of these demon kings is Sirzechs Lucifer from the high school dxd.

In other words, Sirzechs Lucifer and Gremory Clan including Rias exist, but Issei Hyoudo does not exist

In general, characters from other animes and novels make up the characters of this world, but each one has a new background story and follows a new plot and storyline.

Of course, the characters will maintain their appearance and personality, and their family relationships will also exist

Also, keep in mind that the main character is overpowered from the very beginning of the story, and his powers are based on his character in the game he was playing on the earth just like Overlord, so you can expect some elements from Overlord as well.
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Synopsis

The realm of the gods

Realm of the Gods virtual reality game had easily become the most famous game in the world after a few months of its launch

Not only the players could become god-like beings, but the game gives the players the most possible freedom

Arthur Zipfel, the heir of the richest family on the planet

He was one of the top players in the Realm of the Gods and is said to have the highest chance to become a God

And in the end, he managed to reach the highest level and by combining legendary and mythic classes succeeded in creating the Dark God class.

A God-class that gives him complete control over death, shadows, and destruction

Arthur thought that by reaching God level, he is become the strongest player in the world

But it didn't take long that three other players to reach God level

That was the time when the game managers introduced the last level of the game

Great God!

But it was not easy to reach this level and there was not even one NPC in this level

but Arthur did not disappoint and instead started to max sub-classes that do not conflict with the Dark God class and merged them with the Dark God class.

And he succeeded!

But why did he suddenly migrate to a world with the fantasy structure of ancient Europe?

"Why are the people of this world so familiar?!"

"wait, isn't that anos voldigoad?

"Anos voldigoad the legendary demon god?"

"Isn't she Rias Gremory?"

"Wait, there's even the monarch of Shadows and monarch of Destruction?"

"Oh, isn't that Mitsuri from demon slayer? How is she here too?"

"That's Raizel from Noblesse!"

"Roxy From Mushoku Tensei? How did she become a professor at Demons Academy?"

"Oh wait what? Ophis-chan?"