This is a classic delve, sending the party deep underground to rescue hostages and loot treasure from marauding troglodytes, complete with a 'boss' encounter with the Slithering Overlord himself! (Or herself, it's a bit hard to tell...). The default version is that the party is hired by the Order of the Invincible Sun to go down there to kill monsters and loot their stuff, but alternative reasons are provided or you may come up with a good one of your own.
The DM's Notes include a synopsis of the adventure, an encounter list, scaling information, a note on where this adventure is set if you are using the Dungeon Crawl Classics default world of Aereth, the promised other hooks to get the party involved and an extensive background for the adventure. It appears that there's a lot going on in this particular underground domain, with the Slithering Overlord himself having been driven out of his original home and having taken refuge here, and rival species contesting space with his own followers. Oddly, the Order of the Invincible Sun is described as a paramilitary religious order, which rather begs the question as to why they don't take care of business for themselves rather than hiring a bunch of adventurers to do it for them.
The underground setting is made up of three distinct 'sub-dungeons' which the party can explore pretty much as they please. There are a lot of unmapped narrow corridors connecting them, they are uninhabited and it's up to you whether the party has to explore them or if you just tell them that after a few hours they reach... wherever it is you want them next. The monsters to be found here are all integrated into their locations, each has a clear reason to be precisely where they will be encountered - often with an ingenious backstory of their own, which the party may or may not find out. Its very existance makes them more believable, however. In flagrant disregard of stated Dungeon Crawl Classics policy, some of them are prepared to chat if the party doesn't attack them out of hand... this raises the adventure to new heights as the party can, if they wish, try to figure out what's going on rather than merely kill every creature they meet and steal their stuff. (Interestingly, this option is found in many of the adventures, but it's particularly noticeable here.) Needless to say, some encounters are out-and-out combat ones, with monsters that attack on sight!
The final area, where the Slithering Overlord lives, has some... unusual denizens, the flora and fauna alike should prove quite a surprise. The Overlord himself is presented as an utter 'Baddie', but you might want to treat him a little more sympathetically. Options for future adventure are provided based on whether or not the party defeats him, and provide scope for some interesting and unique plotlines if you so wish. As a 'dungeon delve' this is a particularly good one!
