My players sold their souls to Mahadi??

So, a few things here Id love suggestions on, but first things first— the title. Mahadi, on the wandering emporium, offers to sell the players various magic goods/services in exchange for their souls (via Asmodeus). Two of my players took him up on this offer. As-is, it looks like this has no real consequence until 17th level… obviously I’m not going to let that slide. I need some kind of monkey paw or some way they’re messed up because of this trade. Any ideas would be welcome!

Second on the docket, one of my players asked Mordenkainan for a task they may complete in exchange for the “use of one wish spell”. Obviously, huge potential there. Mord said he would fulfill the request if he was brought EVERY last drop of blood for one single unicorn. They have since found mooncolor the unicorn, but every player loves it besides the one who wants her blood. IF somehow they manage to collect 100% of mooncolors blood and deliver it to Mord, how might the most powerful wizard in the realm present the wish spell? I think of him as honest, but certainly not “easy”. Obviously I need my players to not break the game, and I’d like some fun caveat besides my already existing monkey paw powers as the DM.

Lastly (for now), some of my players are OBSESSED with setting Gargauth free. From what I can tell, this just doesn’t happen in the written campaign? I’d love to make a way, I love Gargauth as a character, and would love to give him an epic outro/segway. (The players fully intend to fight him if they can get him out of the shield.)

I’m open to any suggestions! Perhaps multiple of these things could tie into each other for solutions? I need ideas!