he erotic women of FGO are possessed by you're choosing. Are they taken over by another servant? An outer-god tired of Ritsuka not getting any action? A fan from our world?

Possession is the main focus, can be M2F, F2F. Servants who are children (Example: Abigail) forbidden, but servants only appear like children but actually older (Example: 1st Ascension Kama) are free game. Possession can be anything, slime, transfer, litereally robbing them of their body and life. Author pending how hard or easy to possess Servants, or how well you can imitate them (Personality, power, skills etc.)

Edit: Story picture is from a hyoui-lover possession doujin. There are a few, so check them out!

https://www.fanbox.cc/@hyoui-lover