The Succubus Girlfriend Game Over is an ending which can occur during the Chad Summoning event. This game over is reached if at any point Eric(a) agrees to any form of sex with Nathan, including a blowjob, or if they become knocked out from trying to force their way out of the barrier. In either circumstance, Nathan manages to drug them with a love potion. Given how the blowjob worked out, and Nathan's aversion to wearing a condom, preferring even the blowjob to sex with a condom, it is likely the love potion was applied directly to his dick.
Once drugged, Eric(a) tries to remember the name of her girlfriend "Natalie", but due to the drug's influence, Nathan manages to convince "her" that the name of the one she loves is "Nathan". As the effects take hold, memories of Natalie are quickly locked away in her mind. To cleanly sever the past of his new girlfriend, Nathan renames her to "Sophie" and gifts her a necklace preventing magical tracking from locating her.
Sophie quickly settles into life as Nathan's girlfriend, easily winning over everyone she meets through her cheerful personality and demonic powers. While Nathan is happy enough to have a girlfriend, in truth things are going less well for him. Sophie with her demonic powers is easily the dominant one in the relationship, and as she slowly sucks on his lifeforce, he falls more and more under her control.
As Nathan declines, Sophie grows in power. She ends up seducing numerous lovers on the side to satisfy her where Nathan cannot, and they lavish her in gifts and wealth. Sophie however always maintains time for her master, though she slowly makes adjustments to his mind and body to better fit her subconscious desires. Tattoos of an angel and demon wing on his back, longer and brighter blond hair, blue eyes, an innocent laugh, and enormous breasts...
Bit by bit, Nathan is changed and wiped away in totality, even his memories disappearing, until eventually Sophie bestows "her" the new name of "Natalie" as she takes her girlfriend away from her former life, unknowingly doing the same thing as what was once done to her. Eventually, through the notes left behind by Nathan's uncle the alchemist Gillermo, she would even succeed in turning "Natalie" into a demon.