9 months ago, two of my friends wanted to see what I did with this prompt from writing-prompt-s on tumblr.
This is the result.
Have fun!

Yukio reloads his gun and shoots another demon. The mission is wearing him down, it's been going on for ages already and the teenager is far past his limits, which says a lot considering he usually only ever sleeps four hours a night anyway. What irritates him more though is that he has a test in the morning he hasn't even had the time to study for. He has gone on so many missions lately and it was starting to affect his grades. He hasn't fallen asleep in class yet, but if it continues like it had the last month, that is sure to happen too.

"Watch it, scaredy four eyes!"

Shura jumps in front of him, slaying the demon that just tried to attack him.

"Next time you daydream, do it in your bed!"

There are bags under her eyes and she looks pissed. Shura rarely looks anything but flawlessly attractive, but seems like even she is tired. There have been so many overly aggressive demons lately that everyone in the Order has been working more or less nonstop.

"Sorry, Shura," Yukio apologizes half-heartedly as his headache starts to act up again.

"Don't apologize, idiot! Concentrate on- are you okay?"

Yukio smiles softly.

"Sure," he says and drops to the ground screaming.

What he experiences and what shapes him is pain.

It tears at his heart, his soul and ruins him.

He's drowning, drowning in its endlessness.

He struggles for breath, for life.

It feels just like being consumed by flames.

His body is torn open and healed again a hundred times.

Blood leaves his body together with all the strength he could ever muster up.

Blue eyes, not his own, yet so similar.

Waking up has always been a strange experience for Yukio. He's slow to open his eyes, blurry pictures haunting him and the taste of another world lingering on his tongue.

This time though it's different.

Yukio wakes up with a gasp, staring at the white ceiling above him and breathing in air tasting like disinfectant, all while remembering in great detail how his body has been torn apart and nobody was there to care.

"Yukio? Yukio, are you awake!?"

His father comes into his range of vision, worry seemingly having aged him another ten years.

But Yukio just tries to sit up and get a look at himself.

"You had us so worried!" Shirou says, not making any move to let go of Yukio again.

The teenager doesn't mind. Perhaps he should be out of the age where he needs his father to hug him and tell him that everything will be alright again, but right now he doesn't care. It had hurt so, so much. He's never experienced something like that before and he never wanted to feel like this again.

"That pain-"

"I know," Shirou interrupts him, finally letting go of Yukio. He already misses the comforting warmth.

"It- it wasn't a demon, right? I know there were no demons that could do something like that on the mission."

Yukio doesn't want there to be a demon that can conflict such horror upon him.

"No, it has nothing to do with the mission," Shirou answers and relive washes through Yukio until he sees his father's expression.

He's troubled, that's clear.

"I thought it would be fine," Shirou says. "Mephisto had promised it wouldn't cause any problems."

"What?" Yukio asks and searches for an answer in his father's unreadable expression. "Dad?"

Shirou suddenly stands up and leaves the room only to come back a moment later with a wheelchair.

Usually, Yukio would protest against the aid – he's an exorcist, he can stand on his own feet – but most of the time Yukio doesn't feel like he just crawled out of hell, ruining his legs in the process.

With a little help, he gets seated in the wheelchair and Shirou pushes him down old corridors. They're barely lighted and look more than just gloomy.

"Where are we?"

"One of Mephisto's places," Shirou replies, which, Yukio has learned, is code for I'll tell you when you're older.

And while he wants to know what it is that has his father looking around almost nervously, certainly uncomfortable, he doesn't ask because he also sees his hands shaking. Yukio always thought his father isn't haunted by ghosts, he has seen enough nightmarish demons for it. Yet, life appears to prove him wrong.

They reach the end of the long floor without another word being spoken. Too easily Shirou opens the heavy door keeping them from the next room.

The room is rather dark and Yukio has to squint to be able to make out anything at all, only a green gleam shines within the room.

Shirou pushes him further into the room and closer to the light source.

"Don't," he suddenly speaks up again, catching Yukio 's attention. "Don't think of yourself as a monster after this. You're my son. You're human."

Yukio frowns at Shirou's strained voice, at the pain his father never lets him see, not even when he has too many broken ribs to count.

Then they are around the corner, and Yukio finally sees the source of light.

It's emitted from a green liquid within a man-sized glass container. A body is swimming within it, scars upon scars and wounds bigger and deadlier than anything

Yukio has ever seen grace the pale skin.

To his surprise though, they appear to be healing.

"The liquid is helping him regenerate while keeping him unconscious and oblivious to his wounds," Shirou explains.

"Who is he?"

Horror dawns upon Yukio, seeing the black tail attached to the body and painfully familiar blue eyes.

"This is- Your mother called him Rin. He's always been inside this tube, ever since the two of you were born."

Yukio doesn't take his eyes off the glass. "Why?"

"His body has always been working like it should but there was something missing. For the lack of a better word for what it is, let's call it 'Ego'."

Shirou runs his fingers through his hair, exhaling slowly. He must want a cigarette for hours now.

"Despite what you might think based on the number of demons here in Assiah, the universe strives for balance. So while your Ego has been living in Assiah for fifteen years now, his grew up in Gehenna. That is until he learned how to crawl out of that place."

Yukio feels strangely numb like he is swimming in an endless ocean, waiting to find a piece of land and finally return to himself.

He understands his father's implications, even what he doesn't outright say it.

"And what now?"

"Now we wait for his wounds to heal and then- then we make a plan."

Yukio isn't stupid. He knows that twins share every sensation, that the death of one almost always kills the other.

He's aware of that the same way he knows that his opposite is strong and that they won't be able to keep him strapped down.

It means that Yukio's life is no longer his own, that he has become leverage against a person whose roots are in Gehenna.

He wants to scream, but he fears water will enter his lungs and drown him.

Thanks for reading!
I'd love to hear what you think!