Underfell but, every
time Frisk dies, a flower grows on some part of their body and if they try to take it off, they die. Frisk is 18+ in this AU though. When Frisk meets Sans, Sans kills Frisk many times, but after several loads, Sans stops and join Frisk on their
journey and then at the end of an emotional journey and many loads later, the flowers consume Frisk and Frisk tells Sans to use their soul to break the barrier and then Frisk dies, OH ITS SO
SAD 😭