Neverwinter Nights: Hordes of the Underdark is the second expansion pack for Neverwinter Nights. New features included new prestige classes, tilesets, monsters and the ability to craft items. The key new feature of the expansion, though, was the introduction of epic levels, which allowed players to theoretically achieve level 40.
The story follows on some time after Neverwinter Nights: Shadows of Undrentide. The player character from that campaign has emerged from the Plane of Shadow with a relic which, when used, transports you to a place inhabited by a creature known as the Reaper. He will bring you back to his realm if you die and send you back to the place where you died. Also, by expending the inherent magic in a rogue stone you can "bind" a place to the relic, allowing you to transport yourself to that place from inside the Realm of the Reaper. Every attempt to get rid of the relic is futile.
Answering the call for heroes to save Waterdeep from an assault of Underdark creatures emerging from the Undermountain, you have travelled to the city and arrived at the Yawning Portal, where Durnan gives you a free room. After a strange, vivid dream involving a Drow priestess being told that you were the one who was going to defeat her according to some supernatural source, you wake to the sight of a dark elf thief stealing your equipment. You kill the thief but your equipment is lost, having been teleported away somewhere. Durnan tells you that the drow have been assassinating prominent people in the city. The inn is then attacked by drow and duergar, killing most of the patrons before you can drive them into the portal room, where they are defeated. Moments later, a beholder levitates up the well and knocks you and some of the others down. Four adventurers (Tomi, Sharwyn, Daelan and Linu from the original campaign) charge off down the well after it. A cleric of Sune resurrects the dead and gives you a magical rod that will bring back the quartet who rushed off, in case they should have run into a trap and perished. The kobold bard Deekin turns up and offers his services; since he was short-changed by the publishers when he sold his original book about you, he wants to make another one. He also thinks that you would be the only person to accept him after the bad treatment that he'd received locally.
The player character then enters Undermountain and traverses three levels of the dungeon, potentially finding and raising the four other adventurers - who become available as henchmen - settling a dispute between a tribe of Ogres and a group of Fey (one way or another) and discovering the reasons for the recent attacks. You find out that a Drow calling herself the Valsharess (the priestess you dreamed of) has captured Halaster and, using the Silence of Lolth as a distraction, somehow conquered the surrounding Underdark, creating a vast army out of its inhabitants. She has now begun an invasion of the surface world, starting with Waterdeep. You are also told by a dark elf named Nathyrra that there is a resistance movement and she wants you to help as you are the only thing that the Valsharess fears.
You find your equipment and, with the help of Nathyrra, assault the base where Halaster is being kept. You break the wards holding him when a clone of Halaster arrives, just too late to help. While the two mad mages argue amongst themselves about who is the real Halaster you pipe up and are immediately placed under a geas to stop the Valsharess' plans from succeeding before he teleports you away, accompanied by Nathyrra by her request.
You arrive in the Temple of Lolth in the dark elf town of Lith My'athar, where a fragile alliance exists between the native Lolth-worshipers and a group of Eilistraee followers led by The Seer, a powerful Eilistraeean priestess who explains how the Valsharess has gotten this far. It seems that she has somehow bound the arch-devil Mephistopheles, the lord of Cania, into her service. The Seer tasks you with weakening the Valsharess's armies before she makes her inevitable strike on the town. You can achieve this by bargaining with or destroying her allies. When the attack finally comes, the Valsharess's force is weak enough for you and the defenders of Lith My'athar to handle. You are then to lead the assault on the Valsharess' fortress, but the Valsharess commands Mephistopheles to teleport you to her throne room alone. He does so and you find yourself alone, trapped in a magical cage. She orders him to kill you but he refuses, killing her guards and releasing you from your cage. You kill the Valsharess and Mephistopheles reveals his betrayal. Apparently, the Relic of the Reaper was a plant. His and your fates were intertwined with the relic and by ordering him to kill you, the Valsharess broke her pact with him. Mephistopheles banishes the player character to Cania and prepares to start his own conquest of the surface.
Back in the Realm of the Reaper, you find that neither you or your companions (who were killed by Mephistopheles) can bring yourselves back to life on the Prime Material. The Reaper explains things, saying that Mephistopheles knows his true name and thus is in complete control of him, and has banned him from allowing you to go back to Toril. If you learn the Reaper's true name too, however, then you could command him to let you back. He tells you to walk through a door behind him, one that previously barred your passage. Upon doing so you find yourself in Cania and soon discover that Mephistopheles is using the souls here to re-animate the corpses killed in the Valsharess's war in order to take over Toril so that he can turn it into the new lowest level of hell, putting him above Asmodeus in the infernal hierarchy. In addition to discovering the soul of Aribeth de Tylmarande (who can then follow you, either as a blackguard or a paladin depending on your choices) you find out that the Knower of Names, a former lover of Mephistopheles, betrayed and imprisoned by him on this plane, would know the Reaper's name. You set out to find her, braving this hell's painfully cold temperatures and other dangers to free her and learn the Reaper's name. You speak it to him and he sends you back to Toril whereupon you find that Mephistopheles has made it to Waterdeep. You defeat Mephistopheles, either in combat or by using his true name if you managed to find it out, and once you've beaten him are lauded as the savior of the world.
Planes: Hell • Prime Material Plane
Regions: Cania • Underdark • Waterdeep
Settlements: Drearing's Deep • Lith My'athar • Waterdeep • Zorvak'mur
Geography: Undermountain • Shaori's Fell • Isle of the Maker
avariel • beholder • demon • deva • devil • djinn • dracolich • dragon (blue) • drider • drow • duergar • formian • frost giant • goblin • golem (mithril, flesh, stone) • grig • harpy • kobold • mind flayer • mummy • ogre • rakshasa • vampire • vrock • worg
Baldur's Gate • Baldur's Gate: Tales of the Sword Coast • Baldur's Gate II: Shadows of Amn • Baldur's Gate II: Throne of Bhaal
Icewind Dale series
Icewind Dale • Icewind Dale: Heart of Winter • Icewind Dale: Trials of the Luremaster • Icewind Dale II • Icewind Dale: Enhanced Edition
Neverwinter Nights series
Neverwinter Nights • Neverwinter Nights: Shadows of Undrentide • Neverwinter Nights: Hordes of the Underdark • Neverwinter Nights 2 • Neverwinter Nights 2: Mask of the Betrayer • Neverwinter Nights 2: Storm of Zehir • Neverwinter Nights 2: Mysteries of Westgate • Pirates of the Sword Coast • Infinite Dungeons • Wyvern Crown of Cormyr • Neverwinter Nights (AOL game) • Enhanced Edition
Dark Alliance series
Baldur's Gate: Dark Alliance • Baldur's Gate: Dark Alliance II
Eye of the Beholder series
Eye of the Beholder • Eye of the Beholder remake • Eye of the Beholder II: The Legend of Darkmoon • Eye of the Beholder III: Assault on Myth Drannor
Pool of Radiance series
Pool of Radiance • Curse of the Azure Bonds • Secret of the Silver Blades • Pools of Darkness • Pool of Radiance: Ruins of Myth Drannor
Savage Frontier series
Gateway to the Savage Frontier • Treasures of the Savage Frontier