The legend of the end of time.

After the great war of the seven gods, only one kept watch over the entire world.
He woke up, observed and took care of a hidden equilibrium so that nobody knew his existence or that of the other gods.
They all only lived in sagas and legends and so also in this one.
The last god's name was Kurotsuki.
He was the youngest among them and therefore mentally unbalanced.
And so the wish grew in him no longer to be alone.
And with every year that passed, the desire grew stronger.
So strong that he couldn't take it anymore.
He shared his power, tore part of his soul from himself and planted it in a newborn girl that she would grow up on and keep him company.
But there was a war among the living and Kurotsuki soon had to realize that he had made a serious mistake.
Because the child was spoiled and with it part of his immortal soul.
And the girl became the host of incarnate darkness.
Indestructible, like a god, yet locked in the bodies of mortal girls.