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A young Cait sith(humanoid cat) Travels outside of its village to collect herbs but the ground opened up and exposed a underground cave that was home to A parasitic slime it Immediately tried to take over the Cait sith's body.
But halfway through the struggle it's main core was broken and since they were half fused the rest of the slime changed the cats body into that of a slime that could now Change from fur to slime and add working body parts like wings, thicker tail,and large claws.
By devouring creatures trapped underground and using their parts for the herself and gained a human form from a recently deceased adventure as the cave she fell in was connected to underground dungeon.