The Mermaid Sex Slave Game Over is an ending which occurs if the party loses a fight in Return to Mermaid Caves against any enemy except the Nereid.
Greeted to another day by the Mermaid Princess, it shows Eric(a) who by this point is already too broken to even finish a sentence without breaking into tears. The princess has subjected them to such conditioning that they not only enjoy being used as a sex toy, but can't even remember their name. Worse, their hatred for their current captor is slowly fading away even while the joy of being given a new name by the princess makes them feel sick to their stomach. Being given the task of pleasuring the council alongside the mind-broken Pussylicker, or placed as public sex slaves or even worse punishments, Eric(a)'s story ends on a tragic note.
Maybe one day Marianne will rescue the Cutie Knights and restore their broken minds, but that is a story for another day.