Prologue

No one knows when it appeared.

Perhaps it existed in a distant era long before.

Perhaps, it appeared yesterday.

A door.

A black arched door made of a material akin to steel. However, when advanced analyzers scan, they find this element not of this world.

Moreover, its molecular arrangement boasted a completely new lattice.

The first person to discover the door was Mr. Clough.

A Westerner, a merchant, to be precise,.

Clough and his caravan had to detour; only to find the arch in the middle of the same road.

Sudden, Mysterious.

56 meters high, 18 meters wide, with two tightly closed doors carved with symbolic patterns and sigils.

When Clough pushed the seemingly heavy but weightless door, against the wishes of his men, he had unknowingly opened a new era.

After the door opened, dim light peeked from inside, reflecting vague silhouettes of mountains and peaks.

Clough made an incredibly crazy decision to explore the new world!

This made him the greatest personality of the new era, without a doubt.

Despite opposition, Clough smoothed the resistance. Under the high bounty, a temporary exploration team appeared.

Led by Clough, this exploration team consisting of guards and laborers marched into the door.

There were a total of eleven people, 

three days later, they returned.

But only two of the original team.

Clough and a guard appeared dazed and in a terrible state. Anyone blessed with sight could see that they have had awful experiences on the 'other side'.

When asked of the trip, Clough said nothing, and just had the door closed.

For a long time, he never mentioned the 'other side'. It wasn't until two years later that he wrote this passage on paper.

"... It is a world against common sense, where hope and despair coexist, and light and darkness play together. There is unimaginable wealth there, but demons guard them. Those dark people, those blasphemous beings that disrespect the gods. They are deceitful and cunning. Don't expect to defeat them. How can a drop of water flood an ocean?"

Despite the widespread circulation of this warning, people still continued to explore the world behind, after all, Clough did bring back things more valuable than gold.

They were lighter than gold and even more colorful. They were called "devil's treasure" and sold at astounding prices!

Thus, the door named after Clough became more well-known.

Legends of the Inverse Universe, the Dark People, and the Devil's Treasure continued to emerge...

And so, 37 years later, the second Clough door was discovered.

Behind the door was a brand-new Inverse Universe.

As a result, people had to number the Inverse Universes.

A thousand years later, a total of 127 Clough doors had been discovered!