Just wait for him outside the final chamber, then when he/she comes in, all confused as to why i'm not preparing whatever hell hole of a contraption it is he/she expects, throw a hefty bag of money at the feet and just say: "Ok. You win. You have taken the lives of literally thousands of my men, my pets and my friends. I don't even care anymore. Take what you want from my doom fortress, just leave and dont come back. I know i wont be."
Or alternatively, just spring some mindlessly out of character crap. Like, using magic for the whole of the game, then suddenly switching to just traping them in a cage and shooting them.