Demonomicon is a source book covering demons and the Abyss for Dungeons & Dragons 4th Edition. It is written by Mike Mearls, Brian R. James, and Steve Townshend.
The first chapter of the book, Demon Lore, maintains a great balance between story elements and rules material. It offers a background history for the Abyss covering from before its creation through recent centuries. This history introduces Obyriths into the new version’s lore and covers everything from Tharizdun’s attempts to control the Abyss to Asmodeus’s fall from grace. After the history overview, the rest of the chapter consists of a bunch of small 2-4 page sections offering demonic lore and drop-in game material such as alternate powers for monsters, skill challenges, and hazards. These small sections are my favorite part of the book because they offer great tidbits to build adventures and campaigns around.
The book’s second chapter, titled The Abyss, is primarily a detailed look at specific layers of the Abyss. The Plain of a Thousand Portals, the Blood Rift, Azzagrat, the Iron Wastes, Abysm, and the Barrens each get a long write up. These offer a few pages on each realm that detail major locations, environmental hazards, and the history of the layer. Nine more layers get brief sections consisting of a couple paragraphs of information. Then the chapter has two sections that seem to fit better with the first chapter: Abyssal Portals and Demonic Temples. Like the toolbox sections in chapter 1, these two sections offer background information mixed with drop-in rules material. Finally, the chapter closes with two delve adventurers. The Wasting Tower is a 20th-level delve where the party ventures into the tower of Khin-Oin on the behalf of Shemeshka the Marauder. Mouth of Demogorgon is a 25th-level delve where the party enters into a remote demonic temple in order to close a portal to Abysm. It is somewhat disappointing that both of the delves provided are relatively high-level particularly since the book contains a lot of material that makes it easy to use demons earlier in a campaign.
The third and final chapter offers a collection of demons. Stats are provided for a few demon princes as well as many different types of lesser demons that cover a good range of levels. The demon princes with stats in the book are Kostchtchie, Oublivae, Pazuzu, Phraxas, and Zuggtmoy. This noticeably misses the big three demon princes of D&D (Orcus, Demogorgon, and Graz’zt), but that is understandable because those three have appeared in previous books for fourth edition. The artwork for this chapter is superb even if a few pieces are re-used from previous books. There were also a couple monsters entries that seemed a little disconnected from pre-existing D&D lore. For instance, Clockwork Horrors are now demons and the Nabassu entry seems to ignore the existence of the Nabassu Gargoyle in the Monster Manual.
Overall, Demonomicon is a fantastic resource for any Dungeon Master planning to run an adventure or campaign focused on demons or the Abyss. The toolbox elements in the first two chapters are excellent and I hope to see more material like that in future books from Wizards of the Coast. For Dungeon Masters that already own demon-focused source books from older editions, the fluff material is likely going to be largely duplicate, but I think the new and updated rules are still enough to make it worth the price.