From the publisher's website:
Devils may seek to corrupt your faith and mind, and demons certainly look to twist your body and destroy your home. But the fiends known as daemons seek something altogether more fundamental from mortals—they seek life itself. Evil without the distraction of binding law or capricious chaos, they look to the world with hungry eyes for fresh souls to consume.
Horsemen of the Apocalypse continues the Book of the Damned series by taking a detailed look at the daemons—ushers of catastrophe and harbingers of disaster. Personifying the worst ways a person can die, daemons constitute the third arm of major fiendish races, hailing from the nightmare realm of Abaddon, where they are led by four legendary figures—the so-called Horsemen of the Apocalypse.
In this 64-page book, you'll find:
Presentations on the Four Horsemen themselves—paragons of death, famine, war, and pestilence
A complete list of all of the major daemon races, including details on how to summon and manipulate them—if you dare!
Rules for what daemons do with human souls after they harvest them but before they consume them
Information about how daemon cults work, including several new spells and magic items associated with these despicable organizations
The souldrinker prestige class, a daemonic cultist who has learned how to harvest mortal souls to further his own power
Eight brand-new fully statted daemons to plague your characters