I picked up Against Tsathogga from Frog God Games on sale for $2. I did this on the strength of Fog God Games materials, the lure of a high-level Swords & Wizardry adventure, and on the giant frog monster promised within. It's meant to let you use a limited edition monster mini called Tsathogga by Center Stage Miniatures.
This is a short adventure meant for high-level S&W play - it's aimed at 4-6 characters of level 12+. The short version is that a tunnel complex in a vile swamp is the center of a magical ritual to bring forth an elder frog god-thing and it's up to the PCs to stop it.
Oddly, the module comes right out and says that a) the PCs have NO chance against Tsathogga, but can prevent him from appearing, and b) fudge to prevent them from preventing him from appearing. I'd like to read this as "show them the big scary mini but it doesn't materialize and kill them" but equally it could mean just fudging so they have to fight the unstoppable monster.
The adventure assumes high level PCs, and puts in very nasty diseases, miasmas, and other impediments to success. The swamp interferes with health, recovering spells, and rest in general. The short version is that from the start of the move into the swamp the resources of the PCs are going to be worn away and recovery is unlikely to keep up. This potentially makes for a swift pace to the adventure as slow exploration works against the finite resources of the PCs.
The adventure has a very good conceptual use of a ticking clock - the challenge isn't to kill some foes, but to beat a clock during a fight. Very nice. It's a little unfortunate in that the clock starts ticking at the start of the fight, so it's "X turns after Y" not a fixed time. So in fact it's to the benefit of the PCs to avoid rushing to beat this kind of clock - it won't start if you wait. The adventure handles that with environmental difficulties that can and will encourage speed of adventuring, as mentioned above. Given a choice, though, I'd go with both the environmental challenges and an actual time limit, so there is a benefit to cutting a few corners and rushing headlong into danger to earn yourself just a few more moments to deal with the threat. Right now, the only push is the cost in resources.
It's a very liner adventure - not a sandbox so much as a direct go-and-raid-this linear adventure. The dungeon is equally linear. There is all of one choice, and both are merely short forks. Each room is challenging, although some seem pretty much arbitrary in their challenge. One room limits magic, for example, but the limitation starts and ends in that room. It would feel better (and probably be more ominous) if the limitations built up and got worse as you went into the dungeon, so you feel as if the resources you had available were getting cut away. Instead it just feels like, "Don't use X to bypass obstacle Y in room Z, but you can use X normally elsewhere."
There are some nasty fights in it, though, and the link between fights and the clock make for a tense setup.
The adventure has a couple of new monsters - the Custodian of Tsathogga and a Degenerate Ranan. Both are quite nasty and interesting. The Ranan come with a promise for more stats on their culture in another supplement, although which supplement isn't specified.
One major issue for me is that many (seemingly most) of the monsters encountered in the adventure are only described Tome of Horrors IV. At least one other is in Tome of Horrors Complete and a further in Monstrosities. Still others, I have no idea where they are - they aren't in the S&W main book nor in the Pathfinder Bestiaries (since there is a Pathfinder version, I decided to check, just in case it was written and then converted back.) You get stats, but missing details which may or may not be important - I don't own those books, so I can't tell you either way. At least one monster can summon other monsters, which is not statted and I don't know where to look for them.
As someone unfamiliar with some of the monsters, this was very annoying. I feel like I got crippleware, because now I either need to make stuff up (which I can do, but I don't expect published adventures to require that for core monsters), or buy another book - and $15 is a lot for a PDF just to check out a few monsters for a $5 adventure. I'd be fine with this if the page for the adventure said, "Tome of Horrors IV required" because I'd have gone into it knowing this, not thinking "I'll flip to end and find their stats" but not have them.
How is it for GURPS?
It's not bad an an adventure framework for GURPS Dungeon Fantasy or an equally high-powered straight fantasy game. GURPS PCs would have a lot more ways to solve the problems within, thanks to a much more diverse set of spells and access to advantages like Resistant to Disease and rest-free recovering energy pools. But it would be easy to bump up the lethality and difficulty to make them suffer to the level appropriate for this kind of "save the world ASAP" high-level romp.
Overall, the adventure is very good. It's appropriately lethal for high level play, and it's extremely rough without being unfair. Some of the limitations on spells and powers seem a little arbitrary (only in room such-and-such) instead of broadly applied (from room such-and-such and deeper in the dungeon).