The Cthulhu Mythos is the fictional universe and continuity that has come to surround the darkly mysterious tales of horrifying eldritch entities and ancient Outer Gods originating in the early 20th century writings of HP Lovecraft, later expanded by, and co-opting in, many other writers. It has made several appearances in Atlantean publications. It can be seen as overlapping with the Yellow Mythos.
See, for example, Monomyth 51 or Arkham Monologues.
Atlantean Publishing publish a series of Mythos-inspired poetry broadsides, Xothic Sathlattae. For details on submitting material to future instalments, and other Mythos-related calls for submissions, see Current Requirements.
See also Necronomicon.