The Drow Trilogy, also known as the D Series due to its official coding (D1-D3) are an Adventure Path for Advanced Dungeons & Dragons 1st edition in which the party ventures into the Underdark of Greyhawk to confront the machinations of the drow. They are canonically a sequel to the Against the Giants trilogy, and have their own sequel/conclusion in the module Queen of the Demonweb Pits. The entire Adventure Path has been published in the form of the super-module Queen of the Spiders.
These adventure paths consist of three modules; Descent into the Depths of the Earth, Shrine of the Kuo-toa, and Vault of the Drow. Confusing things, Descent and Shrine were also republished in a combined format, also named Descent into the Depths of the Earth.
The plot of the original modules Descent Into the Depths of the Earth and Shrine of the Kuo-Toa places a party of player characters (PCs) on the trail of the drow priestess Eclavdra through the Underdark, a vast subterranean network of interconnected caverns and tunnels, battling various creatures on their journey.
In the last module in the preceding G-series, Hall of the Fire Giant King, the PCs were supposed to have discovered that the drow had instigated the alliance between the races of giants and their attacks on neighboring humans. The drow that survived the party's incursion have fled into tunnels leading deep into the earth. The adventurers will have arrived at the bottom of the dungeon below the cave-castle of King Snurre.
D1 Descent Into the Depths of the Earth
The PCs seek the home of the drow by traveling through an underground world of caves and passages. In the tunnels, the adventurers first fight a tough drow patrol, and the next major fight is with a raiding party of mind flayers and wererats, who have halted their patrol long enough to torture their drow prisoner. The characters also find a grand cavern containing drow soldiers, purple worms, a lich, a clutch of undead, a giant slug, sphinxes, trolls, bugbears, troglodytes, wyverns, and fungi.
D2 Shrine of the Kuo-Toa
The party continues to pursue the drow. The party encounters a kuo-toan rogue monitor who helps them cross a large river for a fee. A party of Svirfneblin (or deep gnomes) approaches the player characters on the other side, and the party has a chance to convince them to help them fight against the drow. As the party travels, signs of the drow are all around; the drow are allowed to pass through these subterranean areas, even though they are hated and feared by the other local intelligent races. The party then moves through kuo-toa territory, ruled by the Priest-Prince Va-Guulgh. If the PCs appease the kuo-toa and respect their customs, the evil kuo-toa are not openly hostile to the party, but will attack if the party gives them a reason. The party learns that the drow and kuo-toa trade with each other openly, but the kuo-toa hate and fear the drow, resulting in frequent skirmishes between the two peoples.
D3 Vault of the Drow
After traveling for league after league into the Underdark, the adventurers come upon Erelhei-Cinlu, the vast subterranean city of the drow. The adventure is written in a very open-ended fashion, giving the Dungeon Master (DM) free rein to script any number of mini-campaigns or adventures taking place inside the drow capital. An extensive overview of the drow power structure is given for just this purpose. Eventually, the players may discover an astral gate leading to the plane of the Abyss, leading into the Q1 module, by exploring the Fane of Lolth.
The impact of these modules cannot be understate; they literally defined the existence of the Underdark and many of its most notable denizens. These were the first ever printed stats for the drow, who had prior to that simply been mentioned as an evil, underground-dwelling counterpart to the elf race, as well as the first ever revelation of lore for their goddess Lolth. The modules also provided stats for other Underdark creatures, most notably the svirfneblin.