After some less-than-enthralling Ravenloft reads--and even some Unspeakable Oaths that I wasn't crazy about--this felt like coming home, if your home was a crack in reality in which time is meaningless and a nameless named city arises like a bad star to absorb all cities. (Which is an uncannily accurate description of my home.)
First, the negative: this book bills itself as being for both 6th and 7th editions of Call of Cthulhu, and I suspect that it was produced around about when Chaosium was (a) switching from 6th to 7th and (b) going through financial hardships and structural changes. All of which is to say: the 7th edition books that I've seen are real slick, and this feels less so. This primarily comes up in the art.
Second, all the positive: the book bills itself as "three scenarios exploring Hastur, Carcosa, & The King in Yellow" (the subtitle); but what that subtitle leaves out is that these are three scenarios for different time periods, along with some rules for what it means to be a medieval person who suddenly remembers that they fought the King in Yellow in ancient Roman times. This is like an arrow aimed straight at my heart, as I have long wanted to explore roleplaying through time, where maybe your past selves's actions influence the possibilities open to your present selves. (This isn't quite that, but it's close.)
Third, I don't know that I love love any of these scenarios, but each of them has some truly tremendous parts. For instance, the first scenario takes place in ancient Rome, in a seaside resort city where some rich folks are about to put on a very exciting play. The PCs experience weirdness--gosh, all these slaves who had something to do with the play are going mad, how odd--but are actually out of town for the play, so the adventure is: weirdness, river boat tour, come back to a city on fire and have to rescue your children and maybe put a stop to the manifestation of Hastur. How nice to skip the apocalyptic revelation and just have the PCs there to try to clean up. (That said, I do find it funny that people in the town are talking about the play using the Latin title--Adventis Regis--when presumably everything they're saying is in Latin.)
The second scenario takes place in the Dark Ages, and involves a dying druid who sacrificed all his life to protect people but whose sacred oak was torn down--and now he's crashing Carcosa into Norman-occupied England. What I love here is that the druid is spreading madness, but rather than be generic about it, each manifestation of madness is precise and limited, and the thinning border between England and Carcosa is nicely described likewise. I mean, I really could see making your way through a blizzard and suddenly coming upon some titanic bronze statue that wasn't there before--couldn't have been there before.
The third scenario... I find a little odd: it takes place in the future, in the End Times (as described by one of the monographs that Chaosium produced, which were sort of third-party ideas that Chaosium then prettied up, but which were not actually canon, I think), where the Earth is given over to monsters and what few humans remain survive in a series of hollowed out asteroids. Anyway, the PCs discover a ship acting strangely, have to dive into the ship's virtual dreamspace, and save the children (that are the computer) in this nightmare town (are you following this so far?), and then have another fight with another Carcosan entity. I don't know exactly what didn't work for me here, though it probably starts with me not really buying the End Times setting; and it certainly doesn't help that the PCs have this extra fight at the end when it feels to me like so much of the emotional climax is freeing the computer/brains from the nightmare.
But that said, I really enjoyed this book: it's much more heavy on the adventures than I thought it was going to be, but I enjoyed them all, and I even enjoyed the short sections on what else the cult of Hastur might be up to in a different time period.