By the Thickness of Water

Long ago, there existed a friendship between three immortal beings of great power. They laughed, they played, and they danced. Then, one day, a terrible monster tore through their innocent fun, breaking it until nothing remained but animosity.

Thousands of years later, just when that friendship seems to be repairing, the beast returns. And this time, it isn't going to stop until it has destroyed every last mortal creature on the planet.

Celestia/Discord.