Demons has always been one of the more popular and fascinating foes in the Dungeons & Dragon games. First edition modules and adventures introduce us to the terrors that are Orcus, Lolth, Zuggtmoy and Ilsidhaur. Then came the dark days of second edition where the demons (and their hated lawful compatriots the devils and the neutral evil daemons) were erased from existance.
I still remember the now infamous letter by James Ward about placating mad moms, and the feeble reply by Dragon magazine (oh, you can still use the fiends from 1e if you want to!) to explain this away, one angry player (and I believe it was a woman) shot back in a letter: Why don't you guys just admit you sold out!
So third edition came along and the staff at WOTC was emboldened to go ahead with re-instating the fiends (not before second edition did a hatchet job in re-introducing them and subsequent writers of the game have a heck of a time trying to create a credible continuity between all three editions).
Other companies, especially Green Ronin, publishes their own treatise on the fiends, and it wasn't until now that WOTC published a book solely dedicated on the most popular fiends of all, the demons. One can point to second edition books such as "Hellbound" and "Faces of Evil: The Fiends" and other assorted Planescape accessories... but they deserved to be thrown in the the Wells of Darkness and tastefully forgetten.
The closest book that resemble Horde of the Abyss was the Book of Vile Darkness, and as reviewer Mike Schell said, Hordes of the Abyss came across as a rehash of BOVD and roughly a third of the book is repeated information under the guise of updating the material from 3rd edition to 3.5. Also, add another one quarter (if not more) of the book as being compiled from Dragon Magazine's "Demonomicon of Iggwilv" articles.
Repeated material includes possessions, spells, some feats, clerical domains, and updated versions of demons such as the goristro, armanite (yes, their spark bolt ability is back!), and bar-lgura.
New stuff includes new demons such as the lilitu, the molydeus making its 3rd edition debut, two new subraces are introduced: the incorporeal loumara and the precursor race the obyriths. Given that Erik Mona, who also wrote "Armies of the Abyss" for Green Ronin, had a hand in this, it is obvious that the Lovecraftian obyriths are thinly disguised qlipppoths.
The Black Cult of Ahm is also introduced, a organziation dedicated on the study of demons and a host of artifacts and magic items are connected to them.
14 demon lords are featured and many people complained that they have been "powered down". There is a small section that explain how you can beef up the demon lords and I for one think that help maintain their mystique. You just don't know which version you might be coming up against. Besides it is easier to power up a monster and to lower its CR. Besides the classics such as Demogorgon, Orcus, and Zuggtmoy, the new ones are Malcanthet, Pale Night, Obox-ob and the oft-mentioned Dagon. Kostchtchie finally gets his third edition debut.
There is also section on the layers of the Abyss detailing among others, Lolth's Demonweb, Demogorgon's Gaping Maw, Yeenoghu's realm, Orcus' Thanatos, Zuggtmoy's Shedaklah, and my favorite, the Wells of Darkness where many demon lords and worst things are imprisoned.
There are three appendixes in the back of the book. The first one, "Ruler of the Abyss" gives some interesting glimpses and hints to who and what those demon lords mentioned in the first edition Monster Manual 2 were, althought I do take exception that Baltazo is a fat green skinned dude wearing a military uniform (!) and the storm demon Aldinach is now "Lady of Change" with wooden mahagony skin and needles jutting from her fingertips.
The second appendix is "Layers of the Abyss" (and strangely, Malcanthet and her layer were both left on on both appendices).
The third appendix detailed demons published by WOTC's hardcover (but did not list Dragon magazine's entries such as the cataboligne, the orlath, etc) books.
Finally, there is a list of demons by CR, which strangely included the deathdrinker (which I found out much later is in the upcoming Monster Manual 4), the water demon (but not the other 5 elemental demons from Dragon magazine) and the skurchur (also from Dragon).
What the book do not have are demon-specific prestige classes and feats, which allows you to customize and advance them in unique ways. But it does have the Abyssal heritor feats which gives you more bonus the more such feats you obtain.
Overall I was disappointed about this book because it is really more of a compilation of previously published materials than anything else, and doesn't give you too much of any new. Given that this is but the first Fiendish Codex, there will probably a follow-up volumne.