Okay, I know Human-Tale already exists, but that one focuses on a monster frisk with human versions of the characters in the underground. Reverse-tale has a similar "Humans lost the war" idea, but with Astell, (used with permission from YourLittleVaporeon) as our main character and a whole new group of humans filling the monster's roles. Also, instead of needing 7 monster souls to break the barrier, this worlds barrier can only be destroyed by a extremely powerful monster.
Characters/ who they fill the role of
Astell-the last monster child to fall into the underground, and child of Asgore and Toriel. She can only speak in hands, however none of the humans seem to know this.
Flowey- A soulless husk made up of Chara's memories and dust, shaped like a Flower. She plays the same role as the original Flowey.