Carcosa was first mentioned in the short story “An Inhabitant of Carcosa” by Ambrose Bierce in 1891. A few years later Robert W. Chambers used Carcosa, and a few other locations that Bierce mentions. It was in a collection of connected short gothic horror stories, “The King in Yellow”, published in 1895. Four of the stories are connected by references to a work of fiction also titled “The King in Yellow”. In the stories anyone who reads the this meta-book is purported to go completely insane.
The stories have a very gothic, Lovecraftian feel to them. They are tales of supernatural powers which are just out of sight and the madness that it brings. This little known book has influenced a lot of authors (as well as RPG game developers). Many authors have either mentioned Carcosa or expanded upon the Carcosa mythos. It has been used by H.P. Lovecraft and many writers of the Cthulhu Mythos. Other writers like Neil Gaiman, Alan Moore and George R. R. Martin have locations named Carcosa in their works.
Here is a short excerpt from the book within the book, hopefully it will not drive you mad. You may recognize it from the journal of Dora Lange, one of the characters from True Detective.
“Along the shore the cloud waves break,
The twin suns sink behind the lake,
The shadows lengthen
Strange is the night where black stars rise,
And strange moons circle through the skies,
But stranger still is
Songs that the Hyades shall sing,
Where flap the tatters of the King,
Must die unheard in
Song of my soul, my voice is dead,
Die thou, unsung, as tears unshed
Shall dry and die in
—”Cassilda’s Song” in The King in Yellow Act 1, Scene 2