Dim Carcosa is one of the Nine Known Worlds, an empty and desolate plane containing only a lost city. It is the home of Lady Casilda and the Saturnine, and the origin of the Carcosa Shard in Season 4; and plays a key role as an effective supernatural antagonist in Season 5.
In the later part of Season 5, it is revealed that Carcosa was originally an ordinary city which was cast into another plane by Strahd von Zarovich in his fury at losing Patrina, to whom he was to be married in the city. The city of Daggerford was built where it previously lay, with the Dragonback Tavern built above the cemetary - which was originally built into a feature known as the "Dragon's Back", and known in Carcosa as "the City of the Dead". Strahd still wishes to restore Carcosa, an act that if completed in its entirety would destroy the Material Plane completely.
Bobby Pancakes was briefly transported to Carcosa as a result of his death and re-animation by Lady Casilda in Season 4, although he appears to have been trapped in the plane and not visited the city. As a result of this, he experiences visions of the city of Carcosa in Season 5, including visiting a fairly pleasant Bar And Grill.
Although Carcosa is associated with H. P. Lovecraft, the name was invented by Ambroise Bierce in the short story An Inhabitant Of Carcosa - although this story only refers to it as a ruined city, with no suggestion of supernatural evil (the story's plot revolves around the unknown narrator searching for his home city, then realizing he is a ghost and stands within its ruin). Its most famous usage was in The King In Yellow, a collection of short stories by Robert W. Chambers, which introduced Carcosa as a lost city and also introduced a character named Casilda. Lovecraft read this book and added some references to it to The Whisperer In Darkness, which created the connection to Hastur; the idea thet Hastur was a supernatural monster came from the later work of August Derieth (Hastur was a benevolent god in Bierce, a place in Chambers, and Lovecraft did not state what it was).
The Seven Gates
Season 5 revolved around an attempt to open the Seven Gates which protect the world from the intrusion of Carcosa. These are listed in Murdered a Man in a Band (5.11), although the requirements for them are only implied:
- The lovers are doomed and frozen in sadness; this is satisfied in Season 5 by the magical freezing and destruction of a wedding party, although it is not clear if this specifically is necessary.
- The burning is brighter, outshining all madness; a forest must burn. The Dragon Friends opened this one themselves by mistake.
- The brothers united, though one may be damned; a brother must kill a brother.
- The learning, a tome of lost truth now in hand; in Season 5 it appears that this does not refer to an actual book, but rather to the death of a person who is aware of the truth, in that case Thomas Smedley. The GM notes for Season 5 reveals that in fact the Hofferman Journal was titled The Learning Gate.
- The others, his servants arriving once more; this likely refers to the formation of a cult devoted to opening the gates.
- Returning, the Dark Lord to settle the score; Strahd von Zarovich must die and return from the dead;
- The Ending, the Lost City back on our shore. This seems to refer to some kind of ritual that must be performed by Strahd to lock the portal to Carcosa open.
The GM notes for Season 5 gave alternative names for the Seven Gates, although it is not known what they would have referred to since it seems this version of the Gates was not used:
- The Bramble Gate
- The Shrouded Gate
- The Monstrous Gate
- The Scarred Gate
- The Blasted Gate
- The Unbroken Gate
- The Child's Gate