Masks of Nyarlathotep

Masks of Nyarlathotep, subtitled "Perilous Adventures to Thwart the Dark God", is a campaign published by Chaosium in 1984 for the second edition of the horror role-playing game "Call of Cthulhu". A number of revised editions have subsequently been published.
Description.
"Masks of Nyarlathotep" is a series of five sequential adventures — in some later editions this was increased to six — set in the 1920s that pit the investigators against Nyarlathotep, the Crawling Chaos and Outer God. There are a variety of handouts given to the players at various points; these include newspaper clippings, handwritten letters, business cards and a matchbox. 
The first edition of the campaign spanned five chapters: 
Publication history.
In 1984, while Larry DiTillio was on strike from his job as a television and movie screenwriter, he earned income by writing role-playing adventures for various companies. One such assignment was "Masks of Nyarlathotep" for Chaosium, a five-part adventure for the second edition of "Call of Cthulhu" that he co-wrote with Lynn Willis. A sixth chapter written by DiTillio set in Australia was cut from the final product due to space limitations. The campaign was published in 1984 as a boxed set that included five books (one for each chapter of the adventure), a 4-page player introduction, an index sheet for the gamemaster's reference, and a cardstock sheet from which a number of separate player handouts could be cut out.
Five years later, Chaosium revised the campaign for the 4th edition of "Call of Cthulhu", and published it as a single 160-page perfect-bound softcover book that included eight color plates of scenes from the adventures rendered by Nick Smith, Keith Berdak, Tom Sullivan, and Mark Roland, as well as full color cover art by Lee Gibbons. In this and all subsequent editions, the player handouts previously printed on a separate cardstock sheet were incorporated into the pages of the book, requiring the gamemaster to either cut them out of the book or to photocopy them.
In 1996, the Australia chapter that had been cut from the original edition was included in a revision for the 5th edition of "Call of Cthulhu", which was retitled "The Complete Masks of Nyarlathotep". A version of this for the 6th edition of "Call of Cthulhu" was published in 2006.
In 2010, a new edition returned to the original title "Masks of Nyarlathotep", with both softcover and hardcover editions. 
In 2018, the game was revised and updated for use with the 7th edition of "Call of Cthulhu" and the "Pulp Cthulhu" supplement, and was released as a three-volume slipcase. New material included a new chapter set in Peru.
Reception.
In the April 1985 edition of "Casus Belli" (Issue #25), Martin Latallo questioned the implausibility of ordinary people suddenly uprooting themselves to risk their lives, noting "most of these encounters and discoveries are fatal in themselves, which presents the central problem of the campaign: what can five unfortunate investigators do against an organization which covers the globe and which enjoys the support of the Old Ones? These modules constitute without a doubt the most deadly campaign ever created by Chaosium, where the stakes are enormous, the dangers immense, and the rewards meager." However, he concluded with a strong recommendation, saying, "An excellent campaign despite everything."
In the May-June 1985 edition of "The Space Gamer" (No. 74), Matthew J. Costello was impressed, saying "The project is, in sum, massive and largely successful. Perhaps only TSR's "Dragonlance" series rivals it in size. And while some flavor may have escaped from this ghoulish stew, there's much here to delight Keepers, Investigators, and admirers of H.P. Lovecraft."
In the November 1985 edition of "White Dwarf" (Issue #71), Phil Frances lauded this book as "undoutedly the finest... supplement to come from Chaosium." Frances believed the series of adventures presented several possible problems, including a plot that was too complex for beginning gamemasters, and the high mortality rate among adventurers due to the extremely deadly adversaries. He concluded by giving it an excellent rating of 9 out of 9, saying ""Masks Of Nyarlathotep" is an adventure that is fun for both sides to play... This is the best campaign I have ever seen, from Chaosium or other sources." 
In the January–February 1986 edition of "Different Worlds" (Issue 41), Willam A. Barton called it "the finest scenario pack ever designed for "Call of Cthulhu". Or for any other game, for that matter." He thought that "the amount of material packed into "Masks"s slender box" would "keep a "Cthulhu" campaign going for months." Barton did warn that this was far too complex for either new players or an inexperienced gamemaster, but concluded by giving the game a perfect rating of 4 out of 4, saying, "an excellently conceived and executed scenario pack that will challenge the most experienced "Cthulhu" players to the utmost."
In the June 1990 edition of "Dragon" (Issue 158), Jim Bambra reviewed the seconed edition of the game and said this book included "some of the most powerful and deadly adventures ever written." Bambra lauded the "superb narrative and the tantalizing clues [that] carry the Investigators along at a cracking pace." He concluded, ""Masks of Nyarlathotep" is the epitome of "COC" adventures. It contains detective work, action, and indescribable horrors in all the right quantities. The writing and editing are first class; the adventures perfectly capture what "COC" is all about. No one should be without a copy of "Masks of Nyarlathotep.""
According to game historian Shannon Appelcline, "Masks of Nyarlathotep" is considered by many critics to be one of the best roleplaying adventures of all time.