Hordes of the Underdark continues the story of Shadows of Undrentide. While in the plane of shadows, the player character finds an artifact called the Relic of the Reaper, that transports them to the Realm of the Reaper if they are mortally wounded, preventing them from actually dying. The player character escapes the plane of shadows, and stays for a while in Waterdeep. Soon, it is discovered that the drow, residents of the Underdark, are using the Undermountain under Waterdeep in an attempt to invade the surface world. Since the Undermountain is supposed to belong to the wizard Halaster, who hates the drow, an ex-adventurer named Durnan asks the player character to investigate.
With the help of a good drow named Nathyrra, the player character finds Halaster imprisoned by the drow. Upon being freed, Halaster informs the player character that the drow queen, the Valsharess, wishes to rule both the Underdark and the surface. He then asks the player character to kill the Valsharess, placing a geas on the player and his allies to ensure that they obey. The player character is then taken to the city of Lith My'athar, populated by good drow, where the player is given more information. The player character is told that the Valsharess has made alliances with various races of the Underdark, and has somehow enslaved the archdevil Mephistopheles. After forming alliance with other nearby races of the Underdark, the player is approached by the Valsharess in a dream in order to turn the player to her cause. Afterwards, she attacks Lith My’athar, and with help from the player character, they either are able to push her armies back to her lair, or the good drow of Lith My'athar are slaughtered. The player character is then captured by Mephistopheles and placed in a cage in the Valsharess’s throne room (if fighting on behalf of Lith My'athar), or brought there by invitation (if he or she betrayed Lith My'athar to the Valsharess). Either way, the Valsharess soon turns on the player character and orders Mephistopheles to kill him or her. The archdevil instead releases the player character, who must then fight and kill the Valsharess. Mephistopheles then informs the player character that he intends to use the armies of the Valsharess to conquer the surface world and turn it into another hell. He then kills the player character so they cannot stop him.
The Relic of the Reaper transports the player character to the Realm of the Reaper, where he/she discovers that the Reaper is under orders from Mephistopheles to keep the player character there. He cannot go against this order, because Mephistopheles knows his true name, giving Mephistopheles complete control over the Reaper. The player character enters Cania, one of the levels of Baator and eventually learns the Reaper's name from the Knower of Names, a being who knows all beings' true names, and is taken to Waterdeep, where the forces of Mephistopheles have already arrived. When the character confronts the devil, Mephistopheles attempts to persuade the player's allies to his cause, before the final battle. Afterward, an epilogue is given for all of the player's companions who survive.
Hordes of the Underdark received generally positive reviews. GameSpot reviewer Andrew Park called it "a solid expansion pack that adds a substantial single player campaign and interesting new options for multiplayer enthusiasts". According to GameSpy, "After the competent, but ultimately pedestrian outing that was Shadows of Undrentide, Bioware came roaring back with this stellar expansion". Chris Chan of New Straits Times thought the graphics were good, but that they suffered from performance problems due when many effects occurred simultaneously. He also took issue with the pathfinding for characters, and found the transition from first person to third person jarring. He said he expected more from the third part of the game's story, and while he enjoyed the game, he felt it was not Bioware's best work. Ure Paul of mania.com comments: "On an overall note, Hordes of the Underdark offers a considerably more rewarding experience than the Shadows of Underntide and, therefore, deserves an honorable place in your RPG collection. It presents enough improvements to the NWN franchise to satisfy its ever-growing community of devoted players. After you're done, you'll most probably get the incentive to try it again with another new prestige class."