This is about how Sebastian became a demon, and it happened when he was still a human being. I did not invent it (or did I?), it came to me in a dream, sort of.
Sebastian Michaelis was an orphan, raised by stray cats. One day, when he was about the age of nine, a beautiful young girl came over to him. She had long blond hair, a black dress, and a pink bow on her head. Her eyes were as blue as the sky, and she was wearing a cross on her neck.
"Hello." She said. "What's your name?" Her smile was as bright as the stars at night.
"S-Sebastian." Said the poor little boy.
The girl noticed his loose and dirty clothes, and brought him with her at the church.
"I'm Elizabeth, by the way. Elizabeth Chloe Middleford" She told him.
Sebastian thought of her name over and over again, so he could never forget it, no matter what.
Elizabeth called a nun, and asked her to clean the boy. Said done, Sebastian was as handsome as new, even for a nine years old. The nun dressed him up in a suit, and gave him back to lady Elizabeth.
"Wow! Sebastian you are so handsome!" She said, all excited. Sebastian blushed.
"May I have this dance?" Asked Elizabeth, as she bowed in front of him.
"O-okay." He replied, blushing as crazy.
Without giving him time to catch a breath, the girl took his hands and they started dancing in circles.
"I'm not so good…" Stated Sebastian.
"It's fine, it's fine. I'm not that much of dancer either." Said Elizabeth.
After half an hour, the two of them were tired. They sat down, and Elizabeth pput her hands together and closed her eyes.
"What are you doing?" Asked Sebastian, not having been in a church before.
"Shh. I'm praying. Asking God favors." Responded Elizabeth.
"Who's God?"
"God created us. He created everything. He can do anything."
Sebastian started praying too, asking for a happy life with Elizabeth.
"Amen." She said.
"Amen." Repeated Sebastian.
"Come with me." Said Elizabeth, grabbing Sebastian's hands and running out of the church, and into the garden. They kept running for a little while, and as soon as they were out of breath, they walked.
"Where are we going?" Asked Sebastian.
"My house!" Replied Elizabeth.
They arrived to a huge mansion, with fountains and statues in the garden and a large entrance. They both entered and the maids took their coats.
"This way, this way!" Shouted Elizabeth, running towards the living room.
Sebastian silently followed her, keeping his eyes on her wonderful hair.
They sat down on a couch and the butler brought them tea. Sebastian was amazed from having tea for the first time in his life.
"What's wrong?" Asked Elizabeth.
"N-nothing. It's just the first food that I've ever had that isn't cat food."
She laughed, and he blushed.
"Hey, Sebby;"
The boy turned red again as he heard that nickname.
"Wanna live here with me?"
"You mean it?" He asked.
Elizabeth nodded. Sebastian stared at his tea for a while.
"Can I bring my cats?"
"Yeah!"
His eyes became wet with tears as he hugged her, and she returned the hug.
"I'll take this as a yes." Said Elizabeth.
Years passed, and Sebastian would fall more in love with Elizabeth every day. They were in their twenties' now.
"I'm home!" Said Sebastian, as he hung his coat by the main door.
"Master Sebastian, master Sebastian!" Shouted the butler, running toward him.
"What's wrong, Charles?" Asked Sebastian, confused.
"It's about the mistress."
Just by looking in his eyes, Sebastian understood everything. Without knowing why, he understood the details of the situation.
He ran outside towards the church, as fast as he could. Elegantly, the speed he ran was amazing. He reached the building in no time. He opened the door roughly, but he was too late. The priest sacrificed Elizabeth as an offering to God. Her corpse was in his arms, covered with blood.
Sebastian was filled with hatred and anger. He was so angry, that his body became covered in fire on its own, but he didn't feel he heat. Wings sprouted from his back, as if nothing.
"W-what are you?" Asked the priest, fearing the creature in front of him.
Sebastian clearly understood was happened to him. His hatred for the church transformed him into a monster. He slowly approached the priest, who was shivering with fear.
"Go away!" He shouted. "God will destroy you!"
But Sebastian paid no attention to him, and he grabbed him neck. He pulled the priest up, and he dropped him on the ground.
"Have mercy!" He begged, choking. But the demon paid no attention. He grabbed his neck once more and flew in the sky, breaking through the ceiling. The priest was struggling to survive, but that didn't help. Sebastian opened his mouth to an unnatural size, and bit off his head, then dropped the body. But he was not yet satisfied. He took the soul of the dead man, which was trying to escape, and ate it whole. The he went back into the church, and ate the rest of the body. Right at that moment, a nun came walking in. As she noticed the winged beast covered in fire, she screamed and pulled out her cross. Sebastian could now not let any people who saw this form survive. He rapidly approached her and bit off her right arm. The nun was crying and screaming in agony, then he ate the remaining body parts and soul in her.
Right after, he let the flames go off and walked over to the corpse of Elizabeth. He could not do anything for her now. He used the little bit of humanity left in him to send her in heaven, but he knew that they would never meet again. He could now never die, and even if he did get killed, he would go to hell.
Sebastian flew away, leaving the town with the bloody church behind. Those who came in after he left would understand what happened, but there was nothing they could do about it. A century later, Sebastian came back to England, and started serving Ciel Phantomhive with contract. The reason he served this precise young man, was because he was the fiancée of the descendant of his beloved Elizabeth, who was also named after her: Elizabeth Middleford.