HOW DO I GET THE EPILOGUE
(MAJOR SPOILERS)
The epilogue is a collection of images without dialogue. It's part of the harem ending, check the official walkthough. Additionally, you need to tell Asmodeus that you don't want to leave anyone behind and you need to follow the path of at least 2 girls, but the more the better. Grabbing the grimoire is irrelevant for this ending, the walkthrough is wrong about this.