3rd person P.O.V. :
"What do you mean I can't deactivate this form?!" Asta said it out loudly while Lucifer thinking how is he going to explain to him. After while of dead silence between them, Lucifer took a deep breath before talking "The day they decided to execute you and you saw your family died which awaken your demon side. You are a child of a human and a devil, it isn't any normal devil, it is a supreme devil. Your demonic side is starting to merge with you, that's why you can't revert back to normal. When you've awaken your demon side, it will slowly merge together with you"
"What will happen to me if the demon side of me and I fully merged togther?" Asta asked "and who are my parents?"(I totally did not plan for the parents one kay? So expect the idea is lame)
Lucifer hesitated but he answered him "I don't know what will happen to you when you and your demon side fully merged together, your personality will probably mix with your demin side and your mother's name is Lichta, her body absorbs mana and life force but she couldn't absorb Liebe or my life force, she was killed by Lucifero who took over Liebe's body for awhile. The only thing is we don't know about your father. I just heard the rumors about a supreme devil and a human had a child 16 years ago. No one knows the supreme devil's name or where he came from but I know you inherited one of his ability."
It is almost night time after Lucifer told him everything he know about Asta's parents, Asta felt tired as he went to sleep. Lucifer looks at Asta who is already asleep, 'When the devil and humans hybrids know about themselves, their other side will merge with them faster. I hope Asta is alright.' Lucifer thought.
***
Asta opened his eyes and found himself in an endless void, the void color isn't red, it is black instead. He saw another him sitting and facing his back at him. The other him is like his demonic form but his hair is white, his horns are more obvious and his wings are folded. Asta also noticed he is wearing usual clothes and without horns, tail and wings.
(Italic words are highlighted as the other Asta)
"You finally know about yourself, I've been waiting for you for a long time" the other Asta said to him as he stands up and walks to Asta. "Why do you look like me?" Asta asks as he slowly backs up "I am you, the difference is I am your demonic side, your anger and hatred" he explained
"Do you know what happens after you know your secret? YOU WILL MERGE WITH ME FASTER! THAT'S WHAT I AM WAITING FOR" he shouted as he rushes at Asta and attacks him by starting with a kick that is aiming at his head.
Asta managed to dodge the kick but he received a hit from the air by his ability.
"YOU CAN'T EVEN USE THE FULL POWER OF YOUR OWN ABILITY! YOU ARE JUST USING A SMALL AMOUNT OF IT! DO YOU KNOW THE MAGIC KNIGHTS WERE USING YOU AS AN ULTIMATE WEAPON, NOT AS A KNIGHT INSTEAD?!" he said as he use his ability to attack Asta and Asta keeps getting hit
"Wait, what do you mean by using me as an ultimate weapon?!" Asta asked and he stop attacking him. "Ho? You don't know about it at all?" The other him is surprised and he sighed"Looks like I need to explain to my other dumbself. But before that... if I defeat you we will merge together, our memories will merge together too. So I don't need to explain it right now"
"Fine but what about we just do a fight without our ability? If I win, you will not merge with me but if you win, then you will merge with me. Deal?" Asta offers him "Deal" he smirks.
They rushed at each other and both of them recived a punch from each other. But the other Asta's punch is harder which made Asta went back a bit. Before Asta could react the other Asta kick him.
After a few times of punching and kicking each other, the other Asta is exhausted. "I won..." Asta said while he is trying to catch his breath. The other Asta smirked and Asta's eye sight started to fade.
"No matter if you win or not, I will still merge with you and the speed will be even faster. So you've lost." He starts to laugh hysterically and Asta blacked out.
***
Lucifer sits beside the sleeping Asta until it is dawn. Liebe came out of the grimore and saw him sitting beside Asta "Hey, what are you worrying about?"Liebe asks
"It's Asta, we aren't sure when he will merge and what will happen to him." Lucifer said while Liebe stay silent
Asta's eyes twitched and he started to awake up. Both of the devils noticed Asta's horns became more obvious and his eyes are telling him that he is seeking for blood.
'Has he merged with his demonic side?' Lucifer thought as Asta looked at him and grins "Finally I've merged with my other self" he said