>>49696957
The idea is that Okina unlocks enough of Rumia's power for her to bypass some restrictions imposed by her seal (but not enough to completely break it since no one wants to risk opening that Pandora's Box). Mai and Satono provide information and the location of Rumia's targets to her, and Ran and Chen fix the leaks in Gensokyo's boundaries from which these entities enter after they are dealt with.
The idea for the fanfic comes from how mysterious Rumia is in canon. No one knows what she does most of the time nor where she is. She doesn't hunt humans and at most just spooks them by turning into a ball of darkness that bumps into trees. Because of that, many think she's just stupid and the shrine maidens don't bother with her, but reading between the lines in the interview Aya gave her and some cracks here and there hint that she hides some greater intelligence and control over her powers than she lets on. So what if she has a secret double life working for higher powers within Gensokyo?
Also a bunch of influence from Hellsing 2000

>>49331823
You find out everything about Rumia was a charade hiding something sinister, but she takes notice of your realization. She stops doing her characteristic pose, her smile wears off, her face starts being obscured by a darkness with only her crimson red eyes being noticeable, and she starts approaching you.