Atomic Underworld: Part Two by Jack Conner

In the world of the Atomic Sea, war is about to explode, and Tavlin "Two-Bit" Metzler, a rundown gambler who thought he had nothing left to lose, is about to find out he has everything to lose. When a series of strange murders begins in the mutant-filled sewers below the capital city of Ghenisa, Tavlin is thrust into a sinister plot involving spies, mobsters, cultists and otherworldly horrors, and if he doesn't figure out who the bad guys are and stop them the entire world will fall under the shadow of the terrifying Empire of Octung.