I have been transported into an erotic novel where the protagonist falls into darkness, so I intend to help the heroine.

Mai fell asleep reading an erotic novel about the protagonist's descent into darkness, only to find herself transported into that world. When Mai woke up, she had become the sister of the heroine's friend in this new world. In a world where magic, demons, and gods exist, will Mai be able to successfully save the heroine from her descent into darkness?