For Frisk, ensuring a peaceful coexistence between humans and monsters is all he ever wanted. However, he never imagined the wrench that an all-powerful AI based off of a sun princess could throw into those plans. While he can tolerate humans leaving behind the earthly life for Equestria, he draws the line at said AI threatening monsterkind's existence.
As such, Frisk loads back to when he left the Underground in order to find a solution to the problem of humans, monsters, and (artificial) ponies coexisting together.
A crossover between the Optimalverse and Undertale. This story is obviously not canon compliant to the Optimalverse due to it being both a crossover and the universe crossing over having magic. Spoilers for both Friendship is Optimal and Undertale, though given the age of those two they should be expected. Also, some personal headcanon for Undertale is included in order flesh out the details of the various characters, as well as how this universe's physics works in regards to magic and SOULs.
Coverart by me.