Monotale is about the cast of Undertale facing inner struggles. There are slight plot changes, and a well thought out color theory to go with this AU.
Undyne is the current overseer of the Underground and was pressured to rule as the new Queen by other monsters when both Asgore and Toriel disappeared–but she instead appointed herself head of the Royal Guard and is simply filling in until they return. Asgore is missing, secretly sealed behind a door in the core while this is happening, leaving Undyne to take the human souls.