Overview:
You're kidnapped in your sleep by a powerful being, taken to their world, and they refuse to tell you exactly why they're keeping you there. What's worse is that they have a history of killing people simply because they're in a bad mood. So don't piss them off.
But it there is a bright side... You're allowed to wander around the castle, and the rest of the Demon Lord's prisoners are all sexy girls. Each of them with their own interests, history, and needs. I'm sure you'll find a way to enjoy yourself...