Easiest puzzle in the game, this is where you meet Lucifer, there you will a giant line of demons, just take one step to the right (or left), go all way up, hit the last demon next to you and talk to Lucifer, here's what you need to say:
Lucifer: Pledge your soul to me and i'll make you my most treasured slave.
Helltaker (You): No, thanks. But you can join my harem.
Lucifer: What are you trying to accomplish? You think you can enslave demons to your will?
Helltaker (You): I can offer coffee, turn based strategies and chocolate pancakes.
Boom, you get the achievement after Lucifer joins your harem.