"Cthulhu fhtagn" are two words of a larger phrase from the works of H.P. Lovecraft. The full phrase, "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn," means "In his house at R'lyeh, dead Cthulhu waits dreaming."
Cthulhu is a malevolent fictional god whom Lovecraft first created in his 1928 short story, "The Call of Cthulhu." The story establishes that Cthulhu is trapped in the underwater city of R'Iyeh. Cthulhu is an enormous being who appears to be a mix between a dragon and a human with an octopus-like face and wings. His worshippers chant "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" in anticipation of his rising from his underwater imprisonment.