Shortened prayer of worshippers of Cthulhu Cult. In full "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" - "In his house at R'lyeh, dead Cthulhu waits dreaming."
Cthulhu is fictional evil deity from stories of H.P. Lovecraft, first appearing in Call of Cthulhu from 1928.
no example of usin cthulhu fhtagn in normal conversation unless you´re a worshipper of Cthulhu.