This is a synopsis/pitch/or something of the story idea I came up with. It could turn into a short story.

A black, white spotted koi fish appeared in a pond at night to greet the white, black spotted koi fish who appeared by day. As nature’s law dictates, the shadows of mortal beings must fade - the white of your eyes should turn to gray, lest the pond be polluted. But when no creature with sharp whites or lashing fins appeared, an Arbiter, an inhabitant of the pond, swam out of the water to bring an end to the immortal fish before they destroyed his home. He persuaded two beings, hungry from their travels, to compete for the two pond dwellers. One magical being was a warlock. An ink, redwax and venom mage who, as a child, once turned the moon into a massive cobra because someone took a bite out of his last macaron. The other, an enchantress, who most believe to be the mischievous child who committed the heinous act, is rumored to be an immortal shapeshifter and empath who enraged the warlock further only to correct his anger and take the glory for saving the world along with his favorite snack. 

He spent the night crying while she laughed outside his window. The two faced each other at opposite ends of the pond and cast their lines, along with spells to aid the bait they threw into the water. I won’t bore you with the details of fishing. It was a lot of waiting. That’s all I can say. 

The arbiter threw a heavy sigh. Then, he realized the two koi fish hadn’t moved an inch since the competition began. No one except him could touch them. He knew he would be stuck with these two for an eternity if he didn’t intervene, so he replaced the two intangible creatures with replicas he conjured out of his sleeves.

As he left the two to their fishing, two rivals families confronted the Arbiter, one who represented the warlock, the other the enchantress. They demanded he give one fish to each of them, and since he had cheated them out of their winnings, the families gave nothing in return. The Arbiter, happy to be rid of the fish, gave them away.

The Arbiter took both fish by the tails and intended to throw them into a bag of water. But the second before they entered the bags, they each spat into the Arbiter’s eyes and rendered him blind. Anger rose in the ancient man. He cast a curse upon the two. The two fish would become human over time and never be able to apply the magic they learned so long as they had human hands.

But an even greater anger surfaced from the pond once the warlock and enchantress realized they had been swindled. Limbs extended out of the water and dragged the two magical beings in. Their magic, siphoned from their bodies, fueled the pond as the small body of water turned into an ocean wall. This ocean wall didn’t just separate the world. It split every citizen in two and gave them distinct features: one cottagecore, the other dark academia.

The two wizards would spend the rest of their lives trying to swim out, only for the current to drag them further in. The anger of the pond intertwined with the curse the Arbiter placed on the two fish.

Over time, the two koi fish transformed into two human children. One child grew up in a small wooden cottage filled with charms, living daisies and all the dresses she could ever want. No one knew why she had large eyes as white as pearls, but they accepted her. They called her Lillibeth. The school she went to taught light magic, as everyone on this side of the wall could only learn of the light.

The other lived in a gothic style church with a built-in library of dark tales and all the dresses she could ever want. No one knew why she had buttons for eyes that never moved, but they accepted her. They called her Agatha. The school she went to taught dark magic, as everyone on this side of the wall could only learn of the darkness.

One day, in a shared tradition of fishing both sides took part in, the two girls stared into the ocean for as long as they could. The girl from the cottage marveled at animals made from ink, everyone with long, sharp, black nails and white, scraggly hair and flowing gowns and chains, each with a cross around their necks. The girl from the church saw a poodle with flowers in her mouth, everyone in blouses and corsets 

When they placed their faces into the water, both girls screamed as one of each of their eyes popped out of their faces. The families who took them in found the two girls, each with different types of eyes. Lillibeth had one button eye, one pearl eye. Same for Agatha. 

And they each saw a world divided. Knowing there was something out there, they sent each other letters through the ocean wall. When the time came for magic lessons, the schools that taught Lillibeth and Agatha were shocked to see a rare mutation.

The two could learn both light and dark magic. But they couldn’t teach the girls anything about bringing the magics together, as it was such an ancient practice the spells were lost to time. The two girls, who couldn’t learn magic thanks to their eyes, came up with their own spells as a game of make believe and tested it on fish they caught from the ocean wall.

But what is magic but an expression of the imagination?

One of the spells Agatha and Lillibeth invented ended up turning a piranha invisible. After a short time of experimenting, they each realized they had erased both the light and dark of the animal. Lillibeth, a fast learner, took the spell and fine tuned it. She could now make anything visible; time, emotion, even ghosts. She could heal souls and bring back memories. Agatha, just as fast, learned how to reveal dark, ancient secrets and reverse blindness. 

The two grew famous, but couldn’t use their magic on people. When the Arbiter found out about what they could do to fish, he split himself in two and confronted the girls. He said they must train, grow into sorceresses so powerful that his curse couldn’t hold them back. Once they’re strong enough, they should be able to heal him and he’ll be able to see again. 

But he knew that, once he regained his sight, the girls would transform back into koi fish, the ocean wall would fall and everyone’s bodies would merge together. Essentially, the people who split would die as their bodies combined. He didn't tell them.