This goes against some of the things you set up in the AU, but it was inspired by it. I hope you enjoy the story.
Two tiny human babies slept in their bassinets, already showing signs of who they were going to be. The girl, on the right side of her parents’ bed, lay sprawled, her mouth wide open and drooling, a little smile quirking at her lips as she dreamed. The boy’s bassinet was on the left, and he slept with his legs curled, an arm behind his head as if to defend his vulnerable soft spot, and his expression was remarkably serious for a face so small and soft. Still, they had the same plump rosy cheeks, the same soft sprouting of brown hair, and their heads turned to face their parents, and thus, to face each other as well. Different, but connected.
“They’re so UGLY,” the demon muttered to himself.
He hung in the air above them, near the ceiling, casting the softest golden glow from his form. In the demon world, he could be anything, he could shift through countless possibilities in the time it took to think…but here, he was limited. A simple shape was all he could take…not even just simple, but GEOMETRIC. Ugh. On the bright side, the pyramids were rather striking, so as far as forms went, he didn’t think a yellow triangle was the worst he could have gotten. And his eye was large and dark and handsome, and he even had arms and legs! Some demons couldn’t even manage that much, so he was a bit proud of his stick-like appendages, all things considered.
Of course, he was still planning to get himself a better form as soon as possible…and with these children, it was very possible.
He hummed absently to himself and circled over the children, ignoring the parents in between them. The babies were oblivious to him, the stupid things, happily sleeping and digesting and breathing and drooling and all those other things humans did that made him squeamish. Still, he could sense their potential…these children were rather exceptional as far as their kind went. He needed that potential.
They were both going to do well for themselves, but he needed to get the best result, and that meant the maximum energy. He lowered closer to the girl to get a closer look. “Alright, kiddo, let’s see what you’ve got-”
And a skinny black arm not unlike his own popped out in front of him, blocking him from getting any nearer to the infant. “Shhhh, she’s sleeping!”
The pyramid jerked back and stared at the one who had stopped him. It was one of his kind, and one of similar power judging by the similar form. He had arms and legs, he was a relatively decent size, he had a large eye in his center, and he glowed. But his glow was pale blue, as was his eye and his form, which was a simple block. How dull!
“Besides, she’s spoken for,” the block continued. “Shooting Star here is mine!” He sounded very proud of himself.
The pyramid sighed dramatically and rolled his one eye. “Are you kidding? Ugh, fine. The spare is just as good, probably.”
“He’s not a spare, he’s her twin,” the block informed him, sounding a little like he was scolding the other demon. “Sometimes humans manage to make two children at once. It’s special.”
“Humans aren’t special,” the pyramid replied coolly.
The block seemed slightly taken aback, but he spoke again soon enough. "Well, if you say so. I am called Dot Matrix on this plane. What’s your name?“
The pyramid considered not telling, but his pride wasn’t going to allow this inferior being come up with who knows how awful a name for him on his own. “Name’s Bill Cipher. What are you supposed to be, blockhead? A piece of the sidewalk?”
“I’m a floppy disk!” Dot replied happily, as though it was the most creative thing in the world. Bill was already getting sick of him, maybe he should go look for another potential partner…but he was here already, so it’d be a shame not to take a peek at least.
Bill went over to the boy and found, to his mild surprise, that they had managed to wake the spare up. He squinted at them with tired blue eyes, his mouth bunched up in a pout as he tried to make sense of the beings before him. It was hilarious how little fear the child showed to a creature that would happily tear his forming mind asunder, Bill thought. It didn’t exactly endear the child to him, but it gave as good a first impression as a baby could give.
Bill looked into those eyes carefully, into his soul. “Huh…a pine tree. I don’t think I’ve heard of that sigil before…must be rare.” He considered, scratching at where his chin would be if he’d actually had a face. The baby blinked, and rubbed his eyes with a whine. Even such a small invasion was discomforting.
“You’re not going to hurt him, are you?” Dot asked slowly. Bill rolled his eye again. Just great, this guy was the sentimental type who went and got attached to his humans. He’d probably cause a fuss if he thought his human’s brother was going to get hurt, if only for her sake.
“No, I’m not gonna hurt him. I’ll get more out of this if he lives a while, you know.” After all, Bill could get a decent profit from the boy’s soul now by just ripping it out while it was too weak to fight back, but if this Pine Tree proved to be fruitful, he would gain much more when it matured.
Dot said nothing more, and Bill decided that meant the “floppy disk” wouldn’t get in his way. He leaned over the baby’s bassinet and tipped his little black top hat. “Hey there, Pine Tree! Guess what? Today’s your lucky day! You get to have a demon, and not just any demon…you get ME! You’ll be thanking me when you’re older, trust me.”
The baby just stared at him. If Bill didn’t know how stupid human infants were, he’d think the child looked incredulous.
Bill held out his hand, which was shining now, alit with a heatless blue flame. He placed it within Pine Tree’s reach, and after staring in wide eyed amazement for a moment, the infant reached out his own pudgy fingers and grasped it. Bill gave it a firm shake, and the child did not pull away.
The deal was done. Bill had a human now.