Why Is There a Succubus in the Small Theater?

[Probably the last Seine River fanfic] [Double heroine + double A + N team, the only seedling in the fourth phase] She was once the future mentioned by Entu's team members, and she was a junior who had high hopes. However, that night when the Purple Dynasty was established, the golden palace suddenly collapsed. Familiar seniors quit the group and fell in love. In the blink of an eye, there was no one around them. But this is just the beginning...