What is referred to here as The Carcosa Mythos is the mythos of the King in Yellow as laid out by Robert W. Chambers – including its earliest roots in the stories of Ambrose Bierce – and as built upon by other authors. Although it is essentially the same as The Hastur Mythos explored in Delta Green RPG and the Insylum RPG, the distinction is to take the emphasis away from the Great Old One Hastur and place it more directly on Carcosa and the mysterious King.