Masks of Nyarlathotep is a long-form campaign first published by Chaosium in 1984 for the second edition of Call of Cthulhu, designed by Larry DiTillio and Lynn Willis. It was revised and reissued as a two-volume 7th-edition slipcase in 2018.
Set in the 1920s, Investigators travel between New York, London, Cairo, Nairobi, and Shanghai to pursue the schemes of cults worshipping Nyarlathotep (a chapter set in Australia was cut from the 1984 release and restored in a 1996 revision).
Renowned for extremely deadly adversaries and a high mortality rate, it is remembered as a campaign in which Investigators die one after another.