Tenacity. (Undertale au.)

A girl, unlike the kind humanity could even imagine existing, climbed the mountain. For too long she had been ostracized for being a freak of science. She hadn't meant to fall in, her leg just had a moment of weakness.

A leg, not made of flesh, but of some unidentifiable translucent substance with the skeletal structure on full display.
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Time in the underground and on the surface works differently. The underground exists in its own bubble, magic causing its own interferences with time and space. A long time ago, this bubble did not exist, for the monsters were free on the surface and human and monster alike used magic. But, when humans trapped monster-kind underground, they also trapped the magic.

That isn't to say that those humans, those who undoubtedly mingled and had relationships with monsters in the past, no longer had magic in their veins. It was just hidden.
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This is intended to be a oneshot for now. That might change in the future. This was a really old draft.