In the Gregorian Year of 666 Anno Domini, the previously unknown sky island named "Eden" which was housing the Great World Tree fell onto the surface of Earth. An event named "The Great Regenesis", meaning " The World's Rebirth and Healing".
This massive event catastrophized Earth and all of its inhabitants, and that includes early medieval humans.
The impact of the fall devastated the part of the Eurasian continent it fell on, which was supposed to be futurely called Siberia by the Russians, supposedly a thousand years later. However, due to this event, that history will no longer occur as the world's timelines have been greatly altered.
Instead, in place of Siberia was a gigantic, almost perfectly circular, continental-sized crater that had a similar size to the subcontinent of India.
It seems that "The Great Impact" has greatly reshaped the region, from its previous terrains up to its deeply-rooted native lifeforms, Siberia was now called Udarcrateria, which means "Impact Crater".
Furthermore, the newly-developed and evolved organisms this entire region hosted were more peculiar.
The normal animals and other organisms that once inhabited this region were now wiped out, and in place of them were magically mutated monstrosities with exaggerated features.
Wolves the size of a house, Deers and other Ungulates with the speed and agility of a jetcraft, birds with a wingspan thrice that of a human, insects the size of humans and predatory birds, plants that now wants meat along with sunlight, and magically glowing fungi that replaced the previously abundant trees.
This place were now something akin to that of a Lovecraftian realm, filled with anomalies that would soon overrun and ruin the world's previously balanced biosphere.
And all of this only took a hundred years after the impact to take place, as if a magical force pushed these events together to happen at a faster rate than normal.
However this was all far from done, because as a hundred years passed, the Great World Tree that was once only caged in the floating sky island of "Eden" were now directly connected to the planet itself. This means that a large wave of change will happen once again.
As the gigantic tree rooted itself onto the surface of Udarcrateria, down deep underground, until it eventually connected itself to the planet's central core. Where a mysterious essence flowed outwards and delivered itself channeling amongst the tree's roots.
This resulted in this "Core Essence" flowing out into the planet's surface, mysteriously affecting the lifeforms in contact with it.
Subsequently, the Great World Tree also began ramping up its size to the point that its tippy top touches the exosphere of Earth just above the center of Udarcrateria. With its canopy completely covering the skies of Udarcrateria, plunging the region into an endless night.
This was the reason why Tree-Sized Glowing Mushrooms fully replaced trees in the region as the most abundant sessile organism, though instead of sunlight, it feeds of off dead magically mutated animals and plants, as well as the overabundant "Core Essence" lingering within Udarcrateria.
And as this essence spread through the surface of Earth, tides of time and continuity once again altered, with this new natural force affecting reality itself.
But unfortunately, the change was not over, as a interdimensional portal opened up on the Great World Tree's gigantic stalk, connecting Earth and the Universe to a reality that only existed within the said tree, to a land that became widely known as "Amartia".
And from this gigantic portal, various new lifeforms stepped out of. Lifeforms that are mixed with outright physically impossible creatures, up to beings that are somewhat close-looking to past extinct organisms native on Earth.
These new beings pushed out the mutated animals previously inhabiting the Udarcrateria, which caused a domino effect on both the Mutated Animals and even Humans. As the Mutated Animals That got replaced migrated from Siberia in all direction except for the northern direction as there was only the Arctic Ocean.
The Far out human civilizations who were previously basically too far to be greatly affected by this new natural variable, were now far from safe, as hordes and hordes of destructive monsters came cascading downwards into their fertile and humanly developed lands.
The Far Eastern Civilizations were the first to fall, as their lands were the closest and is connected to the greatest natural highway known as the "Eurasian Steppes", which basically gave the migrating monsters a leisure into invading their lands. The civilizations were destroyed in only just 20 years due to how flat and accessible the Chinese plains are.
The second were the Indian civilizations, which was just a desert and several mountain range away from Udarcrateria. As monsters capable of hiking and running for far distances was able to reach and devastate the states in the subcontinent.
The third was the mighty and severely divided middle East, which was just a sea away from the epicenter of the calamity. Monsters similar to the ones that invaded India crippled and destroyed the Arabian powers in just 50 years at the same time as the Indians.
While at last were the Africans, as right after the Arabians of the North fell to the monsters, the Sahelian and Sub-Saharans were rushed in by more monsters.
It is evident that Africa was a far away land, however there was something that pulled more monsters into their land, and it was the volume of populations.
And in just 70 years, Africa was now devoid of humans due to the lower population density and larger distances between settlements.
All of these events transpired mostly at the same time frame, and it all took about 90-100 years in time.
However, in addition to all of that, the last part of the "Great Regenesis" was yet to occur.
As the interdimensional portal was yet to close.
As feet that wears leaf sandals and others steps out of the portal.