This is a fairly easy trophy. To make a deal with the devil you will have to find a devil room. What's good about a devil room is that you can normally find it after a boss on every other floor occasionally and if you are really lucky there is more than one deal available; however, it comes at a cost. Each deal costs hearts and it shows how many is needed. Keep this in mind so you don't accidentally kill yourself by using all your hearts before the third deal.