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Hidden is an Undertale AU that takes place after the events of a True Pacifist Route. Soon after breaking the barrier, the monsters realized that the humans might not welcome a new species with open arms. Asgore encourages Alphys to continue her Determination experiments, but this time not to break a barrier, but to shapeshift into a human form. Using the melting properties of determination to monster matter, Alphys created managed shapeshifting serum that helped monsters integrate into society.
However, Chara and an unknown character are trying to mess up this unusual timeline, and this time, for good.
There are two different paths, one being the Grand Ending and the other being Genocide by Sans. The Genocide Ending is currently being written, although the Grand Ending will be written in the future.
Sans accidentally kills Frisk, which causes Sans’s downward spiral to insanity, wiping out the camp. The story is still in development.
Other AU interaction is present, though subtle.
So far the rest of the story is unknown, but this will change in the future.