Celestia has always known her sister to be a strange being. Strange being an understatement. Celestia herself, just as any mare does, has her 'fantasies', but when juxtaposed with what Luna considers enjoyable, the things she considers to be extreme seem like foals play. After seeing what her sister is capable of doing within the realm of reality, Celestia gains a curiosity in what Luna might be able to do within the dreams she weaves.

Reading the Prequel isn't really necessary.
WARNING: This is a gore/clop fic. That's right, they're combined. It's a wonderful mixture of the worst things. Read at your own risk.