Proceed to raid the House of Hope, take the Hammer and your contract, and slap Raphael about.
I think a HUGE catch there could be that, say, Raph only promises to help Karlach after you fulfill the contract. And perhaps he actually does, but if there's a catch in that contract (as there probably is), then, well, that's your open bittersweet ending - Karlach lives, but now you're going to have to figure out how to get your soul back in post-game.