Here is my interpretation:
Humanities Greed
The first World depicts a heavy industrialised society that uses human labor (probably slave labor) to harness some from of energy from the „Forbidden Sun“ which is most likely an advance form of geothermical energy harvested from the planets core directly. This energy was used by the ruling class to power there advanced machnies and weapons; which may be used to build space ships or atleast advanced forms of Ark Maxim to colonise the moon in order to mine it form resourcess. The slavery was unbeareble and the poor souls prayed for salvation, they prayed to Nika… and they prayers were heard. Through there reckless harvest of the planets core more and more natural disasters arise (earthquakes, vulcanic eruptions, rising sea level, ect.) which destabilice the world and creates strange weather phenomena until to the point that the world was almost destroyed by a sea of fire and magma which represented as a dragon-like creature (wraith of the God of Earth) and the arise of the Hell Fire Serpent/Blood soaked serpent which was likely a gigantic rift where magma was shooting out and cold down to become the Redline. The ruling class fleed the burning world in there most advanced Space Ship Uranus leaving the rest to die in the sea of fire.
Arise of new Races
Interesting is that in the first world are only humans shown no other races. Could be that all races except for the Human Race be artifically be created or be modified by the remaining scientist of the first world to better adept an changing and deadly enviroment. Giants for there strenght and durability, dwarves that can grew any plant and turn barren fields and turn these in lush forests, Fish-Men and Merfolk that can survive an drowing world and live underwater and the pinnacle of genetic alterations the Lunarians that can survive in any kind of environment. With time the new tribes forgot there origins and the once advanced society that ruled over the world turned into more „primitive“ societies.
A Miracle or Curse?
Was it the blessing of a merciful deity (God of the Forest)?
Was it the prank of a whimsical devil?
Was it the last attempt of a mad scientist?
The birth of the Forbbiden Fruits that compremise the wishes and desires of Humanity.
Birth of the Nerona Dynasty
After sometime the Ruling Class returned because the energy of Uranus got too low to sustain permanent flying. Seeing the destruction of the world the Nerona Family vowed never to use the „Sun“ again and to prevent the destruction of the world again.
Rediscovery of the „Forbidden Sun“
After some time the Moon People come down to the world which had many alterations on themself, small wings to better navigate on the moon with lower gravity. The reason they came back to the world was that the resourcess on the moon dried out. With time the Moon People made contact with the other Kingdoms; the Kingdom of Gods on top of the Blood-Soaked Serpent (Lunarians), the Kingdom of War and there horned Warriors (Elbaph), the Kingdom of Beasts of lived on the back of the sinful Behemoth (Minks), the Kingdom in the deep sea ruled by Maiden who commends the great Leviathans of the world (Fish-Men Island) and lastly the many Human Nations; which one would be later become a Great Nation. The Moon People shared there knowledge about the technology of the past, which one was about the „Forbidden Sun“ and the prospect of infinite energy for all and end pointless wars for resoucess.
Saving the World. Two sides of the same Story
Fearing the destruction of the world knowing through the tales of his parents, grandparents, Imu choose to fight against the Great Kingdom and take away the Sun. Imu with 19 allies fought against the Great Kingdom; both wanted to save the world; both destroyed the world. To fight a monster Imu become a monster in the hopes the save the world but alas three weapons remained and were used tp drown the world. Imu and his allies won the fight but the war continues until he meets him again.