The third and most significant mini-dungeon in Thunderspire Labyrinth is the Well of Demons.
That's a dramatic name. It's got "demons" right there in the title. But jaded players might be too burned out by the repeated disappointments of the Labyrinth to really get excited about it. "Oh boy, demons," they'll be declaring sarcastically as they make their way to what will probably be another laundry list of unexciting beat-'em-ups. Demons are a classic enemy dating right back to the earliest editions of D&D, and it's just plain cruel to tease them if you're not going to follow through.
So it's great, then, that Thunderspire finally delivers.
The Well is crawling with demons. Your players are probably going to see more of the extraplanar buggers in the next ten encounters than they will in any place outside the Abyss itself. Once the players get past a couple of introductory encounters they'll barely be able to go two steps without running into an Evistro or a Barlgura.
This is the promise of 4th Edition, finally made good: Epic Tier encounters on a Heroic Tier budget. Players don't have to wait 20-something levels to go toe-to-toe with the fiery fury of the planes beyond - they can do it right at the start of their careers, and feel damn good about it all the while.
Demons aren't even the best of what's on offer here. The Well features the best-designed encounters in the module, starting with a exotic match-up against a group of grab-happy abominations, continuing to a trio of clever puzzle rooms and concluding in style with two fantastic set-piece boss battles. This is classic dungeon-crawling done well, and it's almost good enough to single-handedly lift the rest of Thunderspire Labyrinth out of mediocrity and into the ranks of the halfway decent.
So what's going on here plot-wise?
The Well of Demons is yet another piece of local minotaur history. In the days of Saruun Khel the Well was a monastery dedicated to Baphomet, demon god of the minotaurs. It included barracks for the minotaur priests, and an inner complex known as the "Proving Grounds" designed to test the dedication of potential Baphomet devotees.
Today, the Well has been invaded by a band of gnolls. Their leader, Maldrick Scarmaker, is intent on rededicating the Well to the gnoll god Yeenoghu. He's obtained two slaves from the Duergar that he intends to sacrifice on Baphomet's altar as part of the rededication ceremony. These, naturally, are the two slaves the players are still trying to rescue.
Maldrick's got some kind of vague deal going on with Paldemar. Paldemar's giving him... something... and in return Maldrick's providing Paldemar with any "artifacts and items" he finds in the Well. That relationship is never really elaborated on in any form that the players can get to grips with, which leaves the Well feeling like yet another disconnected station on the all-stops Thunderspire railroad. Thankfully, the inherent quality here is high enough to stop you from caring.
The goal for players, then, is to overcome the gnoll guards, solve the riddle of the Proving Grounds, and burst into the inner sanctum of Baphomet just in time to stop Maldrick from sacrificing the captives and completing the rededication.
Sounds easy enough, right?