Okay so the set up is rather simple. You will play the part of a member in a particular 'Mafia.' And as you can assume is the catch, surprise! THE SUPERNATURAL IS YOU! But back to the Mafia as that's the whole set up. It's not actually the regular kind, though it is somewhat similiar in its need to hide from society and sometimes use a rough hand. This Mafia hunts down and collects supernatural creatures! Not just for show or to sell, but to train and use them for building an army to conquer the world!
Now back to the members, You! You are the generals, the leaders, the elite of this 'Mafia.' Whether ye be monster or man, all are welcome to this global domination scheme. The leader? Well she's sorta a lazy good for nothing vampire that uses you for her goals. But she's hardly the type to backstab, and she does help some. If you form a blood contract with her, it will awaken POWERS. That's right, not only can you be a werewolf, but a werewolf that SHOOTS OUT FIREBALLS FROM ITS MOUTH. In return though, the obligatory 'master' and 'servant' clause exists. BUT YOU CAN BE A TELEPORTING NARWHAL. ... Well a Narwhal isn't supernatural. It is kinda freaky though. I don't have anything against you if you want to be one. I mean a fish out of water is kinda supernatural. Especially if it's a narwhal man.
Anyways, as to how the RP will be work out, it'll be mission based. However, there's a certain 'strategic' way to go about things. The Mafia has to keep itself hidden from society for obvious reasons, but that isn't to say there's ways around that. Moderation is the key word. It will be somewhat like a game of chess. If it was in the dark. And had monsters. And superpowers. And possibly rivals.
So you can expect some dark and edgey stuff, action, and some comedy if the characters allow. Maybe you could call it the evil version of The League of Extraordinairy Gentlemen.