The horniness in this first chapter feels out of place in the dark setting. Hopefully it's just spice added to the first chapter to hook initial readers.
the MC being a little insane aside, this is like the perfect premise for a gacha game.
Kinda like Kancolle (shipgirls fight unknown enemies from the sea, and apparently drowned shipgirls turned into those enemies) but with bugs/parasites instead of ships.