The Underworld was ruled by the Demon God.

After being sealed by True Ancestor in the Different Dimension(Now called as Underworld), Demon God could only do nothing but wreak havoc in it.

The group of Higher-Rank Devil stands at the top of Devil Ranking which a bit lower to Demon God, One Who Govern Underworld.

They were called Seven Deadly Sin and they were aiming to spread their power to above, The World of Living.

Back in the forgotten days, After the sealing of Demon God, their plan had been stopped by True Ancestor and it's kin multiple times.
Some of them eventually give up on the plan while some is figuring out another way.

At one point, The human suddenly started a crusade against True Ancestor and it's kin.
Seeing this golden opportunity, The Devil started to tempt some kin to betrayed True Ancestor.
Some took the bait while the other with their strong will makes them unable to go astray.

The Fallen(Kin) left the battlefield and went into hiding while secretly helping the Devil with their plan.
The human not knowing anything about the deal behind their back, still going their way in the crusader.
Eventually, True Ancestor went mad and destroyed everything surrounding it with nothing left but just simply a deep pit.

After that, True Ancestor(Wrath) went around here and there on a rampage by destroying anything in its path.
That was a long and scary time to the human race, it was one of their nightmare that is recorded in history to this day.

One day, True Ancestor suddenly went quiet and the human decided to send a scout toward the last place of True Ancestor was seen but what greets them was nothing but desert, The mountain that once stood grandly was long gone or it might have turn into the now desert of sand.

The human came to the conclusion that True Ancestor died by its own power since they are unable to find any trace left of it.
Even The Fallen and the Devil have the same thoughts.

But the truth was that True Ancestor actually went to slumber due to having exhausted its power way too much.
The Fallen is unable to sense it(Because of betrayal and thus being stained) unless they are close by before it went into the coffin.
The Fallen work together with Devil in the hidden society out of human sight.

A group of Familiar didn't join the Fallen Faction even though they have also betrayed the True Ancestor.
They make their own faction and called it "Royalty Household", Only those with 'pure blood' of familiar can join it.
But if they ever saw a 'half-blood', They would go out of their way to kill it.

Eventually, the True Ancestor was forgotten by everyone.
The only trace left of it is in the Record History.

After a thousand years passed, True Ancestor opened his eyes.