Please Tell Me the Title of the Book Quickly (I Forgot the Name of the Heroine)

Quickly travel through the story, the heroine is an ancient ferocious beast, and a small world is a horror game. The heroine helps the hero and the second boy to travel back to the "real" world. It turns out that it was a trap and the second boy died. The heroine shows up in her original form to save the hero. The heroine later loses her memory. The hero starts to look for the heroine in the small world.