Carcosa

Carcosa, domain of the King in Yellow, avatar of Hastur the Unspeakable, Great Old One and master of Yellow Sign.

Carcosa was once a world much like Earth. However, through the actions of Hastur's cult, they manifested the King in Yellow within their pocket of reality, unmaking their world's fundamental forces sending Carcosa crashing through to the Outside--the vast unreality that lies beyond creation. Here, black stars and alien moons hang in the sky above the twisted and corrupted planet. Here, insane entities dance in the streets of fallen cities, singing songs to their Yellow King.

Cults to Hastur, established by emissaries such as the Pallid Mask, seek to bridge space and time and make the worlds of mortals conterminous with Lost Carcosa. Such breaches causes the unreality of the dead city to paradoxically lie atop our own world in ways that defy comprehension. Geography, geometry, time and space mean nothing in places touched by Carcosa. Exposure to the nature of the Outside slowly breaks down and corrupts our own reality, allowing more of Hastur's agents--and even the King in Yellow himself to manifest.

Incidentally, performing the play the King in Yellow is the easiest way to bring Earth and Carcosa into alignment, as by the end of the play the two are one and the same. It's for this existential threat to our world that the Men in Black have destroyed every copy they have become aware of. Thankfully, the efforts of the Earth's defenders has meant that incursions of Carcosa have been few, far between, and generally contained. But more cultists bearing the Yellow Sign arise every year, carrying with them more copies of that damned play.