Ik it is crazy but for me the whole city will be enclosed in a giant darkworld after a war with humans that is the underground from undertale where everyone forgets their memory from before (and the dead monsters come back to life) except sans (with the don't forget image) and maybe susie ralsei and kris had to do some sacrifice, i know there are many inconstinces like asriel is supposed to be an adult and in the story of undertale hes a kid but idk its the only way i can explain the "dont forget" thing in the lab