Once upon a time, there were two brothers: Jigoku and Sora. They never got along and always argued with each other. So Sora ended up being the god of the sky and Jugkou being the god of the underworld.
After the Recovery
The first war had started. Sora and his army, Mizu (god of the sea) and Hagane (god of arsenal) fought Jigoku and Guru (god of ghouls) and his ghoul army fought a war about who would rule the world.
The Second War
Jigoku's army got dominated. They got forced back into the underworld suffering fatal wound but no death, but some died from pain since their medical technology was not advanced enough. It took them 10,000 years to just recover from their own selves but their fighting skills became worse than an average human. They want revenge.
Jigoku's Army gets Defeated and Sealed into Hell
After the recovery, Jigoku started planning to take revenge. He thought out of the plan and started to get ready for the apocalypse by training combat skills.
The second war (apocalypse) has started! Jigoku and his army emerged from the ground and broke the peace that Sora's army was enjoying. Sora didn't expect that moment but he still quick prepared his army quickly prepared for war and fought again against Jigoku's army.
Jigoku gets dominated again, but this time he couldn't escape. So Sora set a spell and sealed him into hell. Whoever has done heavy sins would end up in hell and would get slaughtered by Jigoku.