NOTE: Sorry about the lateness; Blogger seems to be having trouble with scheduled posts lately. Any solutions appreciated!
The Slave Pits is an important encounter, but there's not a lot to say about it.
This is, in a way, the culmination of the Horned Hold. The room holds three pits, once intended as cisterns. One is still filled with water, but the other two have been emptied out to serve as makeshift slave pens. The captives that the players have been searching for ever since they came to Thunderspire are chained up within the pits.
All the captives? No; two of them are missing, sold onwards to a band of rogue gnolls. The goalposts have moved again, and the PCs are going to be dragged by their nose to yet another dungeon to find the last slaves.
But before the players can start planning their next move they need to survive this encounter. The slaves are guarded by a group of Duergar and a couple of Spined Devils.
The Spined Devils are awkward. Sure, the Grimmerzhul Duergar are aligned (as we'll shortly discover) with Asmodeus, Lord of the Nine Hells, and sure, I guess they can summon up some devil guards. That said, one wonders about a Duergar outpost that uses extraplanar monstrosities to guard its helpless prisoners while leaving a gaping hole in a critical line of internal defence. The real reason they're here, of course, is not for reasons of flavour but simply to provide some variation to the rotation of Duergar we've been seeing since we got to the Hold.
Once we've all suspended our disbelief and accepted the Devils, they actually work quite well here. They fly, which lets them hover above the slave pits and avoid melee. They provide a sense of accomplishment to reaching what should, by all rights, be a rather anticlimactic jail cell at the rear of the fortress.
On the other hand, once everyone stops being impressed by seeing their first Devils, it's all a little underwhelming. The Devils are backed up by only three Duergar; with no option to engage the Devils in melee, the party will swarm the dwarves and neuter them before they can do anything memorable. It's then a simple matter of picking off the Devils from range (providing the party can't immobilise them or otherwise put paid to their flight). The encounter does PCs an accidental favour by forcing them to focus fire.
 The captives include Gru, "a goblin who was sold by his fellow Bloodreavers to the Duergar." Gru "can provide no useful information about the Horned Hold or the fate of the other two slaves", and he's not foreshadowed during the players' earlier visit to the Chamber of Eyes. What is this oddly specific named NPC doing here? Is he an attempt to replicate sidekick characters such as Splug or Meepo from other modules? Or is he the remnant of a layer of detail that hasn't made it into the final adventure?