The Against the Giants series (the first adventure reviewed here, and the other two here) wraps with a hook for further adventures within the Underdark, based on the premise that the Giants were backed by the Drow. This leads to the party heading into the Underdark to do battle against the Drow.
D1-2 appear to be, basically, TSR’s first hex-crawl for AD&D 1st edition – coming out in 1979. The Isle of Dread, which is the most commonly cited hex-crawl, came out 2 years later. The adventure sends the party through the depths of the underdark, headed for the Drow capital city – the Vault of the Drow (seen later in D3). They have a map from the hall of the Fire Giant King, which puts the party on a straight shot, but also has them running through a bunch of obstacles along the way. They can take detours to get around some of the obstacles, but they’ll encounter other environmental perils and encounters along the way.
The dungeon master notes in the adventure also provide some actually productive guidelines. “Don’t be a dick” has, in theory, been the rule that most DMs should swear by, but the stereotype for 1st edition, especially for tournament scenarios, is that the players and the DM are in a confrontational relationship. Here, the adventure lays out specific notes that if the PCs engage in reasonable precautions while traversing the environment, then the DM really doesn’t, mechanically, have the justification to screw over the party under these circumstances. This doesn’t stop them from being a raging doucheliner, but the rules don’t have their back.
The adventure has two mandatory encounters, and two optional encounters. The mandatory encounters are at a drow outpost at the start of the scenario, and at the Shrine of the Kuo-Toa at the end of the route. In between are two optional encounters, one with an Illithid outpost, and the other at the settlement of a group troglodytes, bugbears, and trolls, who have come to co-exist due to the efforts of the Drow.
The underdark itself is where the hex-crawl part comes in. The underdark is mostly mapped out, with a few set-pieces marked on the map, and then a whole slew of other points of interest marked which the DM can populate for themselves. The adventure comes with a few stock, unpopulated encounter areas drawn out, which the DM can use as templates for the encounters at those points of interest. It makes for a much better structure for the adventure than, say, B1: In Search of the Unknown, which is effectively an unpopulated dungeon map with encounters that the GM can drop in wherever, and no real set encounters.
In the case of B1, it’s a beginning adventure designed for new DMs who really don’t know what they’re doing. In the case of D1-2, on the other hand, it’s an adventure for higher-level characters, and in theory for a DM (and a group of players) where this isn’t their first rodeo. Further, the adventure itself narratively sets up a conflict that the player can work with to populate the Underdark, and fixed encounters that will work with that narrative. There is the conflict between the Drow and the Illithid, with the players having a good idea where the Drow are from the map they have in their perception, but not necessarily the Illithid – and the additional threat of the Illithid and their Elder Brain that could move on to the surface if the Drow are truly defeated.
Further, there is a cold conflict between the Kuo-Toa and the Drow. The Kuo-Toa and their goddess hate the Drow, but they can’t operate openly against each other, putting them in a situation comparable to, say, the relationship between the Soviet Union and China during the Cold War.
This makes for a really strong adventure framework, which can stand reasonably well on its own, but also provides some room for DM tinkering to make it even better.