MnFE—PL

There was a world named the Endless Frontier.
It was a strange place, where "doors" called Crossgates acted as dimensional barriers, separating and connecting a myriad of alternate worlds:
Lost Herencia—a world with a developed robotics technology
Kagura Amahara—a world of peace where humans and oni live in harmony with the cherry blossoms and maple leaves.
Elfetale and Dunopolis—a mystical land where elves and beastmen(仮) co-exist.
Formido Heim—a land in the skies where demons thrive
Varna Kanai—a place where aquatic life lives at the bottom of the sea
This Endless Frontier blended the various worlds and people together... even the concept of time between the worlds was merged.
But... who created such a world?
The creators of the Crossgates, who also connected the many worlds together, was a being from an estranged land called the Einst.
Nobody knows when or even how these giants appeared.
But wanting to return to their own home, they continued to open the Crossgates for an uncountable length of time.
Through their actions, they connected a multitude of worlds and gave birth to the Endless Frontier.
However, with the passage of time and the Einst being unable to open a Gate to their home world, they dwindled in number.
Panic set in, and they began to lay their hands on the Endless Frontier that they had created.
They shed off bits of their own body, which formed the Mild Keil crystals, across the various worlds, the energy from which would destabilize the Crossgates.
This energy, strong enough to bend the fabric of space-time, caused many of the elves living in Elfetale to flee to neighbouring worlds.
Additionally, holes in space-time, called tremors, started to appear, creating partial pathways and connecting an even greater number of worlds.
Resulting in the formation of this band of travellers.
As ironic as it may be, they defeated the Einst, who had been trying to "correct" the worlds.
With the fall of the Einst, the majority of the Crossgates collapsed.
The amount of power given off when a Gate collapsed was enough to destroy the dimensional boundry between worlds... causing them to fuse together.
Thus, the Endless Frontier was renewed into a single world.
The denizens of the various worlds decided to work together to survive in this new, borderless world.
However...
Known only to a handful of people, worlds which "didn't belong" had also appeared in this combined universe.
And the elves, which disappeared in an ancient battle, have yet to make their return.
—This is, the Endless Frontier
Where our story begins anew.