HybridTale is an alternate universe of Undertale taking place after the true pacifist ending. Several ages ago, a rare species was brought into existence as the offspring between humans and boss monsters. These monster-human beings, known as hybrids, are special in that they are born with two souls: one being a monster soul, and the other a human soul. Hybrids are synonymous with were-monsters, being able to transform between their human form and their boss monster form by shifting the presence of their souls.

When the war began, the hybrids were split amongst each other on what to do. While a few of them were fighting either on the side of monsters or humans, most of them were calling for peace. Eventually, seven of them that lived under the guise of human sorcerers suggested that the monsters be locked away underground by a barrier not only to keep the humans safe from the threat of monsters, but to stop the humans from killing any more monsters, and to end the war altogether. The humans drove the monsters underground, and the seven hybrids created the barrier that would lock them away.

The hybrids planned to remove the barrier in the future, as they believed that if they waited long enough after the war ended, the humans would let go of their fears and make peace with the monsters, but the hybrids could never tell when that was going to happen. They kept fearing that the humans were not ready yet, and that removing the barrier would restart the war, so they continued living among the humans in secret, and over time, they slowly departed from their monstrous-selves, and any ideas of bringing the monsters back to the surface to live in peace were forgotten, leaving the monsters alone in the underground with no hopes of being freed.

This story follows a young hybrid named Nico, and an old one named Red, who will both eventually lead the rediscovery and the revival of hybridkind, each in their own way.

