reviewer bias: I am a freelance writer for Mystic Eye Games, but I have no affiliation with MonkeyGod Games. Likewise, Mystic Eye Games does not have any products that could be considered competing products to Hellstone Deep.
Warning: This review contains spoilers.
As a bit of trivia, it is a little known that fact that I, die_kluge, was the absolute first person on the face of planet earth to purchase Hellstone Deep. I did so at GenCon '03 before the dealer floor even opened. It had just come out, and with my signed copy in hand, I set out to use it as a basis (or, the culmination, if you will) for my entire campaign.
Hellstone Deep is a 104-page softbound module written by Steven Montano (Black Ice Well) that is suitable for a party of 4-6 players of 18th-20th level. According to David Hurd (Mr. Monkeygod himself), the playtesters kept getting annihilated, and what started out as 16th-18th level, ended up being 18th-20th level, and some of the encounters were modified accordingly. Furthermore, it's all 3.5 compliant, so the devils and other beasties presented are already taken into consideration with regards to combat CRs. As you know, many of these creatures were heavily modified from 3.0 to 3.5.
Hellstone Deep is set, like many of MonkeyGod's modules, in a place I like to call Generica. While there are names given to the locations, none of the cities or places described are so ingrained in any specific world so that it becomes difficult to place them into your own campaign setting. The gods and legends depicted in Hellstone Deep can easily be placed into any campaign setting seemlessly.
There are basically three parts to the module itself, broken out into three separate locations. The first location, the Temple of Iron Flames, is an introductory location that is used to define the tone of the adventure to the PCs. Here, the PCs learn that there is a demon that is hunting down a powerful artifact. The location is a fairly interesting, and could easily be used as a side adventure, or as a base of operations. This section is relatively short, and can easily be removed altogether if the DM has a different way of introducting the plot to his party.
The second part is the city of Il'Drazza'Kuul, a mind flayer city set deep within the bowels of the earth. As an interesting side note, MonkeyGod received permission from WoTC to utilize mind flayers and beholders in this module, even though they are not in the SRD. This second section is a much larger section, and filled with lots of potential for much player carnage. Here, the party learns that a long dormant link to something called the "chain of scars" is about to be reactivated. If this is accomplished, it will spell much doom for celestial-kind. Here, the players fight some wicked-nasty stuff, and hopefully find a way to make it to their third destination, Hellstone Deep itself.
The third part details Hellstone Deep - a demi-plane within the negative energy plane. Hellstone Deep is a bizarre, sort of upside-down dungeon where you start at the bottom and work your way up. The place is filled with all manner of trap and vile beasties, and culminates in not one, but two epic battles, the first with a half-fiend fire giant with fighter levels, and the second with nothing less than an evil fallen angel in the form of a half dragon Solar - a CR 25 creature, designed to ensure that even the most stalwart of 20th level parties is aptly challenged.
The book makes good use of dialogue and is never short on information. Most locations have "boxed text", which despite being overly prosy, provides good imagery and color for the locations. The disparate locations are all interesting, and unique, and can easily find a home in any DM's setting. Each location is qualified by an EL descriptor, so that the DM knows up front where the challenges are, and how difficult they might be. One feature that MonkeyGod used in this book that I really liked was the "Description and Background" and "Motivations and Traits" sections for all the major NPCs. These sections should be standard in every module, and provide a clear foundation to help the DM understand where the NPCs are coming from, and what their goals are. A "Quote" section is a nice touch here as well.
All the bad guys have full stat blocks in the back (in fact, nearly half the book is devoted to crunchy bits, stat blocks, and maps), and some are quite complex (as you can imagine any CR 20 character being). There are a number of new monsters, including Bloodscreamers (gargantuan aberration) and Hellish Rippers (medium monstrous humanoids), a couple of new monster templates (because hey, you can never truly have enough), a new spell (switchback, sort of a reverse dimension door), and some new magic items and artifacts. All the maps to all the locations are in the back, and while the maps aren't awesome enough to want to hang on your wall, they are clear and provide a sufficient level of understanding for the locations the party will find themselves in.
The art is nothing fancy, though there are several NPC pieces that are quite good. The layout is clear, and consistent. There are two sections where the ext is inexplicably missing (on pages 6 and 13). MonkeyGod has provided the missing text, and can be found on their official forums at Mortality.net.
Steven's writing here is clear, and at times, witty. This module isn't high on role-playing opportunities, or deep political intrigue. It's a bloodbath, and designed as such. The true strength behind Hellstone Deep is the fact that it can be so easily integrated into your own setting, and could easily be made the basis of a long running campaign, as I am currently doing.
If you're looking for a highly political, role-playing intensive module for your high level party, Hellstone Deep is probably not it. But, if you're looking for an adaptable, complex, challenging, high-level blood bath with unique locations, and a time-sensitive "save the world" plot, then you don't really need to look any further. From giant evil monkeys, to Erinyes Bards, to fallen half dragon solars. Hellstone Deep really does have it all.