Undermirror is an undertlae AU where a scientist named W.D Gaster sends two kids and a tiny robot to another universe using a magic mirror. There are infinite amounts of mirrors which the kids can visit any universe they want. Frisk can't reset in this AU, but Sans can. Charabot is just there to tag along with the two.
Notes:(Hero is technically Sans Jr. from the future, the only difference is his eyes are blue due to him having more HP than his dad.)
(Frisk, like Hero, wasn't born at the time, meaning they never fell into the underground and freed all the monsters.)
(W.D Gaster never died in his AU which also means he never fell into the core.)
(Hero is the grandchild of W.D Gaster, while Frisk is the adopted grandchild)
Since I didn't feel like typing a whole essay for this, if you have any questions don't ask the characters, ask me. I'm thinking about making this a separate blog though it's going to take some time and work.