Karma - One Piece/UnderTale fan-fiction

Long ago, two races ruled the earth in peace. Humans and Monsters.

One day a man, envious of the Monsters magical abilities, kidnapped several young Boss-Monsters and used their souls to make fruit shaped vessels containing one magical power each. These powers would be divided into three basic groups: Logia, Paramecia, and Zoan.

Over the years as more and more Boss-Monsters disappeared, 'Devil Fruits' as they would come to be known as, became widely known, along with how they were made, among the Humans and once Monsters caught wind of this they demanded that the leaders of the 20 largest Human countries put a stop to this, only to find out that they were supporting the creation of the vile things.

The 20 human kings declared war on the Monsters so that when they won they could use as many of their souls as they pleased. During the final battle when the Monsters were about to lose some of the Humans who pitied the Monsters made a barrier around a large underground cavern. The barrier would only let people in so once they drove the monsters into the caverns they could not escape. When the monsters realized they were trapped they then, in a final act to spite the Humans, cursed them so that the creator of the Devil Fruits would always feel the pain of the monsters and so that those who ate the Devil Fruits (since the world is covered mostly in water) could not swim and would lose their strength when in contact with the sea.