Summary: "My, my, this is really extraordinary," Mephisto commented. "Never thought Father's heritage would resurface again." Blue flames, tail, and sharp teeth against Grace, halo and wings. Because some say that Satan once was an angel of Heaven so the idea that one of his children might inherit his Grace isn't far from impossible. Shirou really wished somebody would have told him that though. Then again - how hard can it be to raise a demon and an angel?

Spoilers for those who aren't up to date with the manga!

Basically Yukio is a nephilim, Rin does not care for human etiquette, Shiro underestimated parenting and Mephisto is as helpful as always: not at all. Inspiration fro this story? Religion class, my favorite stylistic device and continued research about angels so yes the bible tags are there for a reason.

Thanks to my friends who helped me stay motivated and let me ramble at them about this.

Consecratio

Apotheosis, Anathema

Figuring out Yuri's location was easier and harder than anticipated at the same time. On one hand, it should have become obvious where she was hiding weeks ago and Shirou had no doubt that Mephisto had already figured it out some time ago. She had managed to slip away from Section 13, and even more importantly, Mephisto's watchful eyes. There was no way the Ba'al's arrogance would allow Mephisto to lose track of Yuri for long.

Sadly, the part that made it hard to find Yuri, was the fact that the area was full with demons. From small ones, not more dangerous than coal tars, to demons big enough to make Shirou wonder how the hell Yuri, heavily pregnant, had come through here unharmed. Then again, she was carrying Satan's spawn so maybe those demons left her alone or protected her even. Not to forget that Yuri had always maintained a close relationship with her summons, who were far more frightening than most demons Shirou had come across in the past twenty years.

"She's close!" Mephisto announced cheerily, seemingly unbothered by the cold.

Freaking demon bastard, probably had a spell around him to keep him warm while Shirou had to suffer. The white haired man really wondered what persuaded him to become best friends or whatever with Mephisto.

They tramped further through the snow until finally reaching a cave. It was guarded by two snowmen, who seemed to be on the verge of tears, and so let them through without any resistance. For a moment Shirou considered it being a trap, but then decided against it. Traps weren't really Yuri's style. She loved showing her opponents how powerful she was head-on in those rare cases she decided to fight.

"Something feels off," Mephisto spoke up again, this time sounding actually concerned.

Shirou turned to his companion and was graced with the sight of an unnerved appearing Mephisto.
That was something Shirou believed to be entirely out of Mephisto's ability to express. Even after the Blue Night he hadn't looked too out of place. Now though there was something seriously worrying the Ba'al. Great prospect.
They turned around the next corner, reaching the end of the cave and so Yuri.

She was tired and exhausted, eye bags very prominent and her breathing flat. She wouldn't make it through the next hours. At least not here, maybe in a hospital with good personnel but not here in the middle of nowhere.

Odd how he first assessed the state of his old teammate instead of the two children, glowing blue and white-

Shirou's eyes widened. He had expected Satan's spawn to be there, that was no secret. The child was entirely covered in blue flames that harmed neither them nor Yuri, or anything really. Their ears were as elfish as Mephisto's and the aura they had- well, it was familiar enough to the other Ba'al Shirou had already encountered to confirm their relation. That the child didn't look as much as monster as expected, well, that one was a little surprising but nothing Shirou couldn't deal with, unlike the other surprise.

"My, my, this is really extraordinary," Mephisto commented. "Never thought Father's heritage would resurface again."

Shirou followed his gaze to the other twin. Extraordinary really seemed to underestimate the whole situation.

"They have wings and- is that a halo?!" Shirou exclaimed.

Mephisto nodded, frowning at the child.

"I have to admit, I didn't see that coming," the Baal continued, still assessing the situation.

"Didn't see what coming? What? That one spawn of Satan is a freaking angel?!"

Mephisto rolled his eyes at him, giving him an appraising look.

"You might want to tone it down, there are children here."

"Tone it down? Samael, I can deal with a lot of stuff, Heaven knows I put up with you after all, but angels are even too much for me-"

"Bickering with your boyfriend again, Shirou?" A gentle voice interrupted him.

Shirou turned to Yuri and, just like he always had, replied with a heated "Bastard's not my boyfriend!"

Growing up together had established typical questions with the same never changing answers.

Yuri's laughter, even if more timid, still sounded like bells.

"That's what you always say but I've never seen proof that you're not-" She interrupted herself with a coughing fit that sounded everything but good.

"Seems like I won't have much more time left, huh?" Yuri said as soon as she had recovered. She then adjusted her hold on the children carefully, making sure to not hurt the dark tail, Shiro spotted only now, of the demonic twin, or the wings of the angelic one.

"The one with the flames is Rin," Yuri told them. "Written like phosphorus. He's the older one."

For a moment Shirou thought Yuri was trying to joke, naming the child that. She had always had a biting humor and been a bit ironic. Then again the components of that name were beautiful too and well, it wasn't like they didn't fit.

"Yukio's just a bit younger. My precious little snow angel. He- he's a bit sick I think," Yuri bit out between breaths, which sounded far too whistling for Shirou's taste. Perhaps he had given her more time than she really had. "Please take good care of them, Shirou. I- I know they'll be great."

He wanted to tell her that he couldn't. He wasn't there to take care of her children, he was supposed to get rid of them for good- and her too if she resisted.
She was defenseless now, it would be so easy to fulfill the mission and go back to being the exorcist he was supposed to be. Still, something was stopping him and he couldn't quite pinpoint what it was. The fact that he could tell that those children had human souls? The memories of a young Yuri running around his teenage self's legs probably weren't helping either. He hadn't been good with the other children back then, still wasn't good with children, and yet she had wanted him to play with her. She'd always been such a weird person.

"She's dying," Mephisto spoke up again, pulling Shirou out of his thoughts. It wasn't like him to let his thoughts drift so much. Maybe he'd taken too many missions lately.

"You could go ahead with your mission now," Mephisto continued, a sly smile on his face just as if he already knew what Shirou was thinking. He probably did, that bastard.

"They have human souls," Shirou said, his grip on Kurikara tightening. "I can't kill them."

Mephisto nodded, though Shirou doubted the King of Time cared about moralities as such. "Well then, I suppose I could-"

"The remains of S13 are no place for them," Shirou immediately interrupted. "You made failed copies there, not humans."

Mephisto crouched down, poking the cheeks of the winged twin. "And where else should they go? They don't look human to anyone but you."

And even to him they didn't look that human, only felt human and that still made a difference to him.

"Then I'll take them," Shirou argued and turned to the twins again, who were peacefully sleeping, unaware of how much the world wanted them dead. "Like I said, I put up with you, this can't be harder."

Taking in a demon and an angel? He put up with homunculi, failed clones, successful clones, stupid researchers, a shitton of demons and Demon Kings on top. This couldn't be any harder, Shirou was sure.

Carefully he too crouched down and reached for the child closer to him, Rin. He hesitated for a moment, afraid the flames would burn him, but then picked the baby up. The blue flames were warm, comfortingly so in the cold. Not at all like those horrors he had seen at the Blue Night. He adjusted his grip, fearing it was too tight and he'd wake Rin up, but the boy kept on sleeping.
Picking Yukio up turned out to be a bit tricky with just one free arm and Mephisto being as helpful as always: not at all. Eventually he managed though, the baby's wings folded together carefully. He had to admit, calm like this the twins were even adorable.

"Now look at this, already being the perfect father!" Mephisto announced, a gleeful smile on his face, far too wide with too many teeth.

It was then that the penny dropped.

"You tricked me," Shirou said, glaring at the demon.

"Guilty as charged!" Mephisto answered, still looking as stupid as before. "There's no way I would take them to S13, my dear siblings would end up knowing and I can't afford that, not with how tense everyone has been lately. I'm not parenting material anyway and I don't have the time to take care of them. But you? Yes, you have lots of time you're currently spending on unimportant stuff that you can put into raising those two. So, what you say? Still doing it?"

Shirou glared at the Demon King a bit longer before redirecting his gaze to the twins. "It's not like I have a choice, do I?"

"Not really, no," Mephisto answered.

"What are we doing with Yuri?" Shirou asked. He could tell that she was still there, but her presence was fading and soon would be gone.

"Leave it to me!"

Then Mephisto lifted his right hand, the movement already familiar.

"Öffnen!" He called, for once not sticking to his usual chant. Seemed like he wanted to make sure that this was secure. [Open!]
Above Yuri a portal opened, slowly coming down on her and taking her to one of Mephisto's many pocket dimension.

"Now that this is done, we're going somewhere warmer!"

Mephisto repeated his one word command and this time the portal took them away from the cold place that gave birth to something that, for now, Shirou considered his betrayal of humanity.

Right so let's get some things sorted out right from the start: I have everything for this story written what takes place between the twins' birth and pre canon. I could write a lot more for this time and if this were the main fanfiction I'm working on right now I probably would. But it isn't so I have to make cuts somewhere.

I will update weekly until I run out of prewritten chapters and then stop posting until I have the next Arc entirely done which I'll then post again in weekly updates.

Originally this was a longish One Shot, I aimed for around 10.000 words. But after I had the first part done and wasn't even close to the twins being 15, I decided to split it into a story with chapters. What this means for you? The chapters won't be long, roughly 900 words each. Sometimes a bit shorter, sometimes a few hundred words longer. Since a lot of world building is fit into this story, feel free to ask questions about that xD

Now that this is said and done, I hope you enjoyed this first chapter and I'd love to hear what you think!