The cover, depicting Charon (the Horseman of Death) is excellent and sets up the flavour of the book perfectly. The inside front cover gives a stylish summary of the four Horsemen, while the inside back cover lists the deities, demigods, and other powerful entities that reside in Abaddon (for each, it includes their areas of concern, clerical domains, and favored weapon). If you need a cult devoted to maggots, accidental deaths, or sewers, this is the place to come! The art design of the book fits the theme really well, though some of the art of particular daemons could be better.
The book starts with two pages on the “The First Daemons” and concisely explains what sets daemons apart from devils and demons. Daemons seek the utter oblivion of all mortal souls, and they consume those souls entirely! They’re not after order and hierarchy (like devils) or chaos and pain (like demons), they just want to feed and extinguish life. In this way, they could be the most fearsome of all!
Chapter One (14 pages) provides a well-written overview of Abaddon. It includes a sort of gazetteer, with locations that beg for dark campaigns to visit like The Menagerie, the ghastly Mere of Broken Angels, and the Oblivion Compass. The chapter has two pages on each of the Four Horsemen. The writing here is evocative puts the reader in the daemonic mood instantly. There’s a dozen great ideas on every page—but this book is definitely not for kids!
Chapter Two (10 pages) has a good discussion of daemons in general, frequently contrasting them with devils and demons. There’s a paragraph or so on each of the particular types, with clear distinctions in their roles and methods. There’s also coverage of “daemonic harbingers” (elite, named daemons that have their own following).
Chapter Three (16 pages) has information of more direct relevance for GMs and players. It talks about daemon cults form but how daemon worshippers are fundamentally deluded, because daemons never offer anything in return! The discussion of the soul trade is very valuable (night hags have been part of more than one adventure I’ve seen) and it even lists going rates depending on the type of soul! A new prestige class (Souldrinker) is introduced; it’s pretty cool, building off a spellcaster chassis with energy drain powers to spend “soul points” to activate special abilities. There’s then some flavourful information on summoning daemons (easy to call but hard to contain!). Several new spells are introduced, some of them filling needed mechanical niches when it comes to transferring souls into soul gems, etc. Finally, there are some new magic items—cool, useful, and *very* evil!
Chapter Four (20 pages) is a great bestiary. After a couple of pages on miscellaneous denizens of Abaddon (like divs and night hags), it goes on to feature two-page spreads on new daemons: erodaemons (pose as family or lovers to cause heartbreak), lacridaemons (their sobs lure travellers into getting lost), obisidaemons (gargantuan genocide monsters), phasmadaemons (fear killers), sangudaemons (blood-drainers), suspiridaemons (suck oxygen out of the area), temerdaemons (personify accidental death), and venedaemons (death by magic). There’s some great story hooks just from the concept of these daemons alone.
Overall, Horsemen of the Apocaplyse is an excellent book, and I really can’t think of a single bad thing to say about it. If you have any inclination to use daemons in a game, it is worth seeking out.