A Carcossa is what happens when a madman dreams... sometimes.
Other times, they are tapped into by perfectly normal dreamers. That's right. Tapped into. Not dreamed. Tapped into.
Carcossas are dreams that have gotten bigger than mere dreams, expanding to nearly their own planar status.
Some of them are intelligent. Some of them can dream.
How does a Carcossa form?
The simple answer? No one knows.
The unsimple answer? Scholars assume that they are formed around so-called "nightmare seeds", which are created every time that a creature has a nightmare. Yes. Every time.
What happens is that nightmare seeds are attracted to dreams related to the basic seed, consolidating and growing and consuming and breathing until the random noise caused by the disparate dreams coalesces into a steady whisper of "I am. I was. I will be.
How do you recognize a Carcossa?
Recognizing one is... hard. They seem to most senses that dreamers, even lucid ones, possess as if the nightmare-world was nothing more than a dream had by a potent Lucid Dreamer. Which, to be honest, is close enough to what they are that it isn't an inaccurate assessment.
But it is more complicated than that; Carcossas are not linked to the normal realm of dreams, instead acting as parasites that feed of dream seeds and lesser dreams. In fact, if you die inside a Carcossa that isn't on the Plane of Dreams at the time, you die. Permanently. As if you had died in real life.
Of course, that isn't the most horrendous fate that can befall you inside one of these world-dreamers.
Basic Carcossa Traits
All Carcossas share a few traits in common.
Firstly, they are built around the original seed; when the whispers started, the seed became, for all intents and purposes, a Dreamer. It deludes itself into believing that it's some shape that is relevant to the original seed. This entity (for it isn't a creature, not exactly; there is at least one carcossa where the central "dreamer" is nothing more than a 'mundane' boulder) is heretofore referred to as the Nightmare Heart. If the Nightmare Heart models themselves as a creature, they are always treated as if they had 23 ranks in Lucid Dreaming, and a Wisdom of 30, for the purposes of what they can do inside a dream; they additionally receive a +10 within their own "realm" on all Lucid Dreaming checks, beyond what other bonuses they can "find". Otherwise, they can be practically anything.
Secondly, the Carcossa, while not very effective in and of itself, is highly contagious. (Insert rulesy bits here. Essentially, they warp Lucid Dreaming checks of "infected" creatures so that they are forced to include some thematic element of the Carcossa in them; a dream that is affected enough by the Carcossa basically collapses into being a part of it, with horrible consequences for the dreamer.)
Thirdly, Carcossas are ultimately fragile; if the Nightmare Heart is destroyed in any manner, the Carcossa begins to collapse into it's component nightmares, and any dreamer currently in it is simply wakes up with nothing but bad memories.
Lastly, if a Dreamer enters a Carcossa, they remember every moment of their time in the Carcossa. Intimately.