Her story began before her birth with her father. Lucifer Nightshade.
The story was well known amongst humans in their Holy book the bible. As the bible went Lucifer was the handsomest and smartest of the Angel's and was God's right hand. However he was not content with his own great powers, he soon grew jealous of God's power and wanted it for his own.
And so upon gathering his followers, he waged war on God but of course God won, and Lucifer was ultimately cast out. His followers where cast out with him and God gave them horns and tails and disfigurements as punishments and thrown into hell. Lucifer's powers and a select few higher angels which would become the higher class demons, prevented then from becoming too disfigured but thier wings where now dark and leathery instead of the white feathers of heaven forever marking them as Banished. As for Lucifer, there was no denying he was now the King of Darkness. He could control his form at will and his true one was terrible indeed to behold, few have seen it and kept thier senses about them to retell the tale as many went crazy.
When he was banished, he stole with him the most beautiful angel and most powerful angel in heaven he could find. He considered this his right. As soon as he touched her down to Hell's charred ground her white wings crumbled and faded into ash and she became the first demoness.
Together they had a child, Sitri a demon with parents of immeasurable powers. She was expected to grow up just as great if not greater than her parents as a siegal of darkness.
However when Sitri was a still child a human found out how to break the barrier between hell and earth and somehow managed to summon the unsuspecting current Queen of Hell.
Gracious and impressed the woman who had summoned her had accomplished such a feat the Queen granted the woman one desire and then she would return home. The woman agreed and made her wish; she desired to be the most powerful human in the world. True to the demons word, after staying with her for a short time the woman was a very powerful figurehead in politics and very wealthy but she still wasn't happy. She soon wanted the demons magic for herself. Going into farther into voodoo and ancient dark magics the woman found a way to drain a demons power and take it for herself.
When the time came for the Queen to leave back to her family in hell as promised, the woman enacted her plan and took the Queens power for herself, reducing the Queen to dust and ending her immortal life. This woman became the first witch.
Lucifer went berserk after his wife's death, often going up to earth for heavy killing sprees on his hunt to find the witch. The witch was clever and he never did find her although he would kill the occasional decendant if they grew careless. He made a law where a human summoning a demon would be required to make a contract where the demon would grant the wish but with the price of thier soul. This also protected the demons as if the human broke thier end of the deal or tried to trick the demon, the demon would have permission to devour thier soul immediately.
A few decades later, while Sitri is barely entering her teen stage as demons age slower than humans; he found a fallen angel that couldn't get back to heaven. He had his fun with her and left her for dead, mangled on the side of the road.
She lived and gave birth to a boy, half demon half angel. She died during childbirth, the demon half of her child corrupting her who was made of entirely pure energy. Lucifer found the boy shortly after humans spread tales near and wide about a baby with black and white wings and violet eyes. No one dared go near it unsure if it was cursed or holy.
He nearly killed the boy, disgusted by him and his angel blood but began to think of using him and the potentials of a halfling. He might even be able to use the child as a gate to heaven.
Rather than thoughts of fatherly love, Lucifer took his son home and named him Sebastian, planning of the power and destruction he could cause him to bring.
The duties of parenting fell to Sitri who took care of her new half brother as her father did not care about either of them beyond thier potential.
Lucifer began giving her lessons on how to be a ruler. Often throughout he'd interweave long sessions of torture where he'd rip strips of skin from her back laughing as she screamed. He ensured she always listened to his word and never disobeyed with that and other creative ways.
He said she reminded him of his wife who was foolish for having allowed herself to be killed. She bore it as long as it was for her brother and was always glad when he went back to earth for his next rampage.
Lucifer came to hear a rumor of a Prophesy about 3 keys. The daughter born of a demoness would became the key of hell. And the demon son born half of an angel would became the key of heaven. The prophesy went that if there was a child of half human half demon he would become the key to earth. The humans called him the anti christ as he would then bring about the end of days when the 3 keys where activated together. This would destroy all three realms. He knew this could only be his children who would have the power of celestials. He set his sights on ultimate power and ultimate destruction.
He went to earth trying to find a woman who would bear his child, the anti christ. However, every single woman died after the act purely from the evil energy he radiated. Only one woman somehow managed to survive him. She manager to hid his child despite being concerned about it being the anti-christ of the stories. She named him Stolas and managed to hide him until around 13 where his powers, slow to come in his half human body exploded out, leveling his small city.
Lucifer was alerted by the explosion of power and found the half demon boy in the middle of carnage and bodies with his half violet, half green eyes and knew him as his ill begotten son of earth. He was impressed by what he had done, despite it being unintentional. And he recognized the trauma the boy was going through having just killed his mother and entire town and fully inteded to exploit it.
He brought Stolas back to hell and waited for the end that never came. He was angry when the keys wouldn't activate and often tortured all three of them in his frustrations and as he did experiments on how to activate the keys within them.
To spare her brothers Sitri stepped up into her role as Queen. She would sometimes go with her father to lay villages to waste and demonstrate her upcoming power. She would sometimes have to hide them for extended periods of time until his rage wore off. He never hit her face because she was to be Queen but he wasn't above anything with her brothers.
After the keys didn't work, Lucifer only wanted his puppet Queen and ordered her to kill them. She refused, offering anything in return. After he tortured her for weeks and she still wouldn't give in he promised if she slaughtered every single person she had ever known or gotten close too then he would allow her brothers to live on earth in peace.
Breaking her heart so she could harden it, she agreed and fully blacked her heart with the blood of her loved ones for the cost of her brothers lives. And then she sent them to earth for hopes of a better future on earth where they thought she abandoned them.
Unbeknownst to to Sitri though, Lucifer was still trying to find his sons to either harvest thier power or kill them. After a while enough was enough. Sebastian was a young adult and Stolas a teen when they came to find Sitri with the plea to help them end his power so they could live in peace. They each had almost came into thier full powers, and so they decided to end thier father's reign forever. Thus began the Hell Takeover.