Robert M. Price described, in his essay “H. P. Lovecraft and the Cthulhu Mythos,” two stages in the development of the Cthulhu Mythos. The first stage, termed the “Cthulhu Mythos proper” by Price, was formulated during Lovecraft’s lifetime and was subject to his guidance. The second stage was guided by August Derleth who, in addition to publishing Lovecraft’s stories after his death, attempted to categorize and expand the Mythos
The Yellow Sign is a fictional symbol or glyph, first described in Robert W. Chambers’ book of horror short stories The King in Yellow (1895). Interpretation of the Yellow Sign created by Kevin Ross for Call of Cthulhu.
It is implied that the subject is peering into an endless void, has just seen something no mortal should ever see, or something so horrifying that he/she will never be the same again.