Ghostwalkers

From New York Times bestselling author Jonathan Maberry, the first in a thrilling series of novels based on Deadlands, a hugely successful role-playing game (RPG) set in the Weird, Weird West.
Welcome to the Deadlands, where steely-eyed gunfighters rub shoulders with mad scientists and dark, unnatural forces. Where the Great Quake of 1868 has shattered California into a labyrinth of sea-flooded caverns . . . and a mysterious substance called "ghost rock" fuels exotic steampunk inventions as well as plenty of bloodshed and flying bullets.
In Ghostwalkers, a gun-for-hire, literally haunted by his bloody past, comes to the struggling town of Paradise Falls, where he becomes embroiled in a deadly conflict between the besieged community and a diabolically brilliant alchemist who is building terrible new weapons of mass destruction . . . and an army of the living dead!
Deadlands is one of the most popular RPGs in history, with over a million Deadlands gaming books sold.