Published by Lamentations of the Flame Princess and available at DriveThruRPG or the publisher's site.
Carcosa is a [weird] science-fantasy role-playing game setting with a sandbox approach: whether heroic or opportune, adventurers of all stripes will find freedom and consequence worth their mettle in a horrifying milieu mixing adventure fantasy, the Mythos, and comic book sci-fi with no punches pulled.
This book oozes with oozes and ideas. Carcosa is a land of waking nightmares in which humanity is the victim of both vile sorcerers’ ritualistic sacrifice and the incomprehensible machinations of alien and/or ancient beings.
The defacto rules system of Carcosa is Weird Fantasy - Lamentations of the Flame Princess, though other Old School Renaissance systems would work as well. Special attention is given to how Carcosa varies from Weird Fantasy (i.e. there are no clerics, magic-users, elfs, dwarfs, and halflings).
The majority of Carcosa is devoted to the Sorcerous Rituals (p41 to p73), Monster Descriptions (p74 to p11), and Hex Descriptions (p112 to p213).
The chapters leading up to Sorcerous Rituals allude to the strangeness of Carcosa both as a world and as a game setting. It is in the Sorcerous Rituals section where the truly horrific nature of life on Carcosa reveals it's true form. Let's look at the ritual “Weird Ascent of the Diseased Slime” (p73):
For eighteen hours the Sorcerer must stand by the diseased pit in hex 2511 while uttering the eldritch chants. At the beginning of the ritual a bound Dolm woman (not a virgin) must be cast into the pit, where the mutating viruses twist her body into inhuman contortions. After the eighteen hours, the woman must be retrieved and thrown upon the ground. The Slime God will ooze up through the earth and envelop the sacrifice’s body.
Each of the 96 rituals are equally evocative, and for the most part could rather effortlessly used in other rules systems; Did I hear someone say Burning Wheel?
Many of the rituals require victims of a specific racial color, age, gender, and/or virginal status. It is these details that humanize the victim of these inhuman rituals. Sorcery is not an easy nor safe path — performing a ritual can age the ritualist. You certainly won't make many friends as a Sorcerer.
Immediately after the Sorcerous Rituals is a section on humanity's other enemy, the monsters of Carcosa. Ranging from elder gods, the great race, dinosaurs, and suckered abominations, this section is details all that goes bump in the night.
One important entry, “Ordinary Beasts” highlights that there are no horses nor ordinary beasts of burden in Carcosa. This bit of information further drives home that on the soil of Carcosa, the toils of man are puny and insignificant.
Slime God, described below, is conjured by “Weird Ascent of the Diseased Slime”, banished by “Descent of the Six Thousand Steps”, bound by “The Ineluctable Name”, imprisoned by “The Sunless Watery Blight”, and tormented by “Transmutation of the Slime God.”
This putrid glob of slime combines all the horrid qualities of dolm pudding, dolm ooze, jale slime, and ulfire jelly. About the only types of weapons effective against it are various hi-tech ones such as lasers, bombs, missiles, etc. Further, 24 hours after physical contact with the Slime God, a saving throw vs. poison must be made. Failure indicates that the victim has been afflicted with a rotting disease that will rot him away at the rate of 1 HD per day until he dies.
Not all monsters have associated rituals, there is no known way to control nor conjure Azathoth and Cthulhu.
Hex 2511 is described below — the location where “Weird Ascent of the Diseased Slime” can be performed.
In damp natural caverns is a shrine to the Slime God. The caverns are filled with puddings, oozes, slimes, and jellies, none of which will attack any of the eight Green cultists of the Slime God, nor their leader, a chaotic 2nd-level Sorcerer who has mastered only the Weird Ascent of the Diseased Slime ritual. Any Sorcerer who will join the cult of the Slime God will be taught the ritual after three months.
Six Deep Ones are entombed deep within the glacier. The bodies may be recovered after a thousand man/hours of digging. One of the Deep Ones wears a weird gold necklace worth 360 g.p. If the bodies are allowed to thaw, they will return to life.
My personal favorite is hex 2510.
A horror-stricken man has already been half-transformed into a fruit tree. He is rooted to the spot, and in two weeks the transformation will be complete. At that time the chaotic Blue 9th-level Sorcerer who conjured the arboreal thing that caused the man’s transformation will arrive with his sensual Purple lover. They plan to feast on the exotic dolm fruit that the tree will bear.
Not all locations are as colorfully detailed, but there is ample descriptions to spur the imagination.
There is an interconnectedness between the rituals, monsters, and locations. The author includes relevant cross-references. This serves not only to provide quick access to the related information, but is useful as an adventure generation engine.
Need a monster? Find one that connects to a specific hex or ritual and read up on it. Think about who would summon the monster? Or why someone would go to that hex.
Need a villain? Find a ritual associated with one or more locations. How are they going to get there? Who is following them there? Or find a ritual that requires multiple sacrifices. How is the villain capturing his victims.
Need a location? Find one related to a ritual and explore what it means for a sorcerer to show up ready to perform the ritual.
While the majority of the book details the Sorcerous Rituals, Monsters, and Locations, Carcosa also provides rules for a simple and straight-forward Psionics system, random mutations, wandering monster tables, random robot generator, random space alien armament, descriptions of space alien & ancient technology.
If Carcosa doesn't smack you in the head with at least a full two-dozen adventure ideas and a half-dozen campaign ideas then you aren't opening your mind to its blasphemous insights.
Carcosa is opinionated and not for everyone. Humanity is not the master's of their domain, but are more akin to infighting animals awaiting the slaughter. It is a complete campaign world with discrete subsystems that can easily be extracted and brought into other games.
Pick it up if you are interested in a very non-traditional campaign setting document that pulses with inspiration.
Don't pick it up if you are squeamish or prefer a traditional D&D campaign.