Who is John 'Stef' Stefanovitch from The Midnight Club and what is their importance?

Stef goes back to work despite his wheelchair, obsessed with the idea of taking down the Grave Dancer. When Stef gets the call that the Grave Dancer has been murdered in a local brothel, Stef jumps at the chance to take the case, anxious to see for himself the body of his nemesis. However, during the investigation of this murder, Stef learns of an organization called the Midnight Club, put together by the Grave Dancer and including the heads of organized crime from all around the world. Stef works with a writer, Sarah McGinniss, in unearthing the members of this exclusive club and stopping them before they can create the largest organized crime syndicate the world has ever seen.