Masks of Nyarlathotep has often been called the greatest adventure ever written for any roleplaying game and such praise is not undeserved. Published in 1984 for Chaosium's Call of Cthulhu, Masks might be called "an Adventure Path done right." Starting with a scenario set in New York City, when an old friend of the Investigators asks for assistance shortly before his suspicious murder, the adventure soon takes on global proportions, with clues pointing to London, Cairo, Kenya, and Shanghai. In each city, the Investigators gather more clues about the actions of a cult dedicated to the Crawling Chaos and seeking to usher in the reign of the Great Old Ones upon the Earth.
Two things, in my opinion, make Masks stand out. First, once the New York scenario is completed, there is no single "right" way to proceed. The players may choose to pursue any of the clues they've amassed anywhere around the world. There's no expectation that the London scenario will immediately follow the New York one, for example. This gives a great deal of freedom to the players, something that's essential in investigative scenarios if they're to avoid feeling like railroads. Second, while there is a "ticking clock" to keep the Investigators moving briskly to defeat the cult of Nyarlathotep, it's a long enough one that they can afford to take their time and undertake their investigations thoroughly. As in most Call of Cthulhu adventures, it pays to be methodical and hunt down every scrap of information you can get your hands on. Knowledge is more than power; it's the key to life and death.
Of course, the flexibility and open-endedness of Masks comes with a price: it's a very complex adventure for the Keeper to run. There are five different "main" scenarios in total, each of which includes many minor and side adventures the players might choose to undertake. Likewise, the interconnections between the scenarios, as well as with the overall arching plot of the cultists, are many and take some effort to keep straight. There are dozens of NPCs too, most of whom have secrets of their own. The end result is a massive, occasionally bewildering adventure that, if handled properly, is arguably one of the most interesting adventures the hobby has ever produced and certainly one of the best ever produced for Call of Cthulhu. Handled by an inexperienced or untalented Keeper, though, Masks is likely a recipe for disaster.
Still, I count myself among the many admirers of this adventure. I called it an "Adventure Path done right" above and I firmly believe that. Rather than having a single path from scenario to scenario, players are free to take whatever route they deem best. Investigative scenarios are very prone to railroading and often founder on a single undiscovered clue. By providing such an open-ended structure, players won't feel as if their actions are dictated by the plot nor should they run into many brick walls. Consequently, Masks has an almost sandbox-like quality to it, making it superior to most other Call of Cthulhu offerings, including Shadows of Yog-Sothoth. It's a very demanding format, both to produce and to run, which is why I suspect so few products have followed in its footsteps. That's a pity, because Masks ably takes full advantage of all the qualities that make roleplaying games unlike most other forms of entertainment, most especially the ability of the referee to improvise based on what he already knows about the overall scenario.
If I have any real complaints about Masks of Nyarlathotep it's that, like so many Call of Cthulhu adventures, it doesn't feel particularly Lovecraftian. Sure, the bad guys serve Nyarlathotep and there are lots of Mythos-related tomes and creatures to be encountered, but the whole things feels more like a pulp serial or an Indiana Jones movie than an exercise in cosmic horror. I'm willing to overlook that, since I'm not convinced that cosmic horror would actually be all that fun to play and because Masks is such a brilliant adventure in its own right, so brilliant that, it's definitely deserving of the accolades it receives in many quarters. Is it the best adventure ever written? That's really hard to say, since tastes -- even my own -- vary, but it's without a doubt in contention for the title and well worth a look if you've never had the chance to do so.