Sympathy for the Devil by Kylie Marcus

In the underbelly of Chicago, the Italian mafia has ruled for over a hundred years. Fighting tooth and nail, they’ve used countermeasures against other gangs, mobs, and the feds to keep their iron grip on the city. Then, one day, a gift lands in their lap, in the form of an old handgun.
The Sleeping Devil.
One pull of the trigger and the gun unloads something far more deadly: the Devil himself. Taking on the form of the patriarch of the De Luca family, the current reigning Italian mob, the Devil quickly offers the bodies of De Luca’s children as vessels for his own. They have one job: corrupt human souls, and build Hell’s numbers.
Doomsday is coming.