Van Richten’s Guide to Fiends is written as the thoughts of Ravenloft’s famous monster hunter, Rudolph Van Richten, with additional gray text that represents game mechanics related to Van Richten’s observations and hypotheses concerning “fiends” (a catch-all category for demons, devils, daemons, etc.).
Van Richten’s musings begin with him receiving a number of esoteric tomes as a bequest from a departed friend. Among these books are a sixteen volume set called The Madrigorian. Though these texts are said to amount to two piles that stand at chest-height, Van Richten manages to digest them over the course of nine days. (I’m guessing the author of this supplement has never had to actually engage in that magnitude of scholarship. Van Richten’s fellow academic, Oelie Farringer, manages the feat in only six!) Though initially dismissed as the ravings of a lunatic family, it is discovered that The Madrigorian is actually the record kept by a fiend who possessed many members of the same familial line.
The supplement offers a number of interesting theories that could come up in-game to explain the existence of fiends. One theory is that fiends are a stage in the development of the lich. Once such a being leaves behind the last of their physical remnants as a demilich behind, perhaps their unholy spirit is reborn as a fiend. Another theory holds that fiends are created through the accumulation from the malignancy of human actions within Ravenloft. It is also suggested that instead of summoning fiends from afar, magical conjurations actually will fiends into being.
Of course, it is also suggested that fiends are an influence from another plane of existence that is beckoned into Ravenloft through the horrific sins of the demi-plane’s residents. This last theory is particularly well-fleshed out with game mechanics; offered within the supplement is a system through which a character can be slowly taken over (both physically and psychologically) by a variety of fiends. There is great potential for both body horror themes and the
conventions of possession and exorcism to enter the game here. The mechanics are similar to those of Ravenloft’s usual Dark Powers checks, but instead of turning into a monster the character is sent to a hellscape and replaced with a fiendish power who is now trapped in the demi-plane.
Unfortunately, this early section of the supplement also evidences its biggest problem—and one of the biggest problems with TSR’s mindset during its contemporary era. While it’s clear that fiends in Ravenloft should be unique, singular beings of immense terror, the text bends over backwards fitting that idea into the already extant framework of demons, devils, etc. (in their sanitized 2nd edition forms). The usual demonic and diabolic types seem a poor fit for
what the supplement is trying to achieve, but it’s shoe-horned in anyway because all of D&D’s settings are supposed to share the same common conceits. The Blood War, in particular, feels especially like a square peg being forced into a round hole in this context.
While I was initially finding a few interesting bits in the early sections of this supplement, the middle section drags on and on. Once the book turns to the explication of the various powers that fiends possess, you realize that what it’s really doing is blowing up what should be a single line description of a power in a monster write-up into a full paragraph (or more) that adds nothing new. For example, "Only hit by +2 or better magic weapons" becomes...thirteen paragraphs of text, including *shudder* in-character epistolary fiction. This is the sort of bloat that pads out much of the 2e AD &D era’s books.
The section on additional powers granted to fiends by the various Ravenloft domains they might find themselves trapped in is at least useful and a bit interesting. But then we hit the section on cults that serve fiends. That absolutely should be fascinating; cults are awesome antagonists. For example, look at the best of Warhammer’s adventures, Runequest’s deeper look at fantasy religious allegiance, or even the cult-centric nature of 5e's published campaigns. But it’s difficult to imagine a section on cults that is more lackluster than what we get here. We learn that cults lure in their prospective adherents, that they swear an oath of fealty to the fiend, and that this oath is corrupting. It’s almost as if the book is trying to describe a cult to someone who has never heard of the phenomenon before.
Worse yet, the example cult that serves "the Black Duke" is so devoid of imagination that there is nowhere to go with it. Even the names of the people involved reek of generic fantasy: meet the rogue Scarhand and Sir Ironhand. (I suppose the mighty mage, Merlin Wandhand, was elsewhere at the time.) The cult is called the Brotherhood of the Whip, which barely has potential, but then we’re immediately told that they are so named because the Black Duke carries a whip. It’s all a bit on the nose...and that's the problem with Van Richten's Guide to Fiends overall.