Cthulhu, also referred to as Dread Cthulhu and the Lord of Sleep, is a powerful extradimensional being classified as one of the Elder Gods.
Among the Laundry, they are generally regarded as being one of H.P. Lovecraft's thoroughly fictional creations, and their possible existence is not taken seriously by at least Bob Howard. However, it emerges during the unfolding of CASE NIGHTMARE GREEN that Cthulhu is indeed real, and has subverted the Laundry's American counterpart, the Operational Phenomenology Agency.
During its takeover of the OPA, Cthulhu manifested an avatar of itself, the Mouthpiece, to direct the OPA in summoning its true form. This Mouthpiece resembled a human-sized wasp, and would grow from eggs laid in a human host to occupy most of the host's body, eventually wearing it as a sort of "second skin".