The game takes place after the events of Euryale's Gambit. Stheno and Euryale join with Ashmedai and with the help of the Succubus Queen Igret Bat Mahlat, portal into Hell. There they meet a strange demon who calls himself "Aden" who agrees to help them. The small group battles countless hordes of demons as they close in on the Demonic Citadel where the Demon Lords are holding Athena imprisoned.

[From Steam]