The Carcosa Mythos

From The Yellow Site

Just to quickly clarify, before I go into greater detail:

What is referred to here as The Carcosa Mythos is the mythos of the King In Yellow as laid out by Robert W. Chambers, and as built upon by other authors. Although it is essentially the same as The Hastur Mythos explored in Delta Green 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.