There are two ways to get to the Slave Pits; one involves taking the passage through the wights and the empty crypts to reach the Slave Pits through a secret door.
The other involves fighting the ogre at the south gate and then stumbling into the chambers of Murkelmor, commander of the Horned Hold.
It's interesting that you can completely bypass the Horned Hold's "boss fight" - and, in fact, my players did, so I've got no direct experience of how this battle plays out. (That worked out fine, though, as it allowed me to keep Murkelmor around for a role in my substantially modified finale to the module.)
Presuming that you actually run this encounter, though, Murkelmor's a bit of a beast. He's a level 7 elite, with the hit points to match, some absolutely savage at-will powers and a rechargeable heal that can be applied to either himself or his allies.
Once again, though, having a name doesn't make Murkelmor a character. He gets a couple of lines of textbook villain rant when the players breach his room, and then enters battle. This is theoretically the climax of the Horned Hold, but the players can't be invested in defeating this guy because they may well have never heard his name prior to getting here. At the point where players run this encounter, they've got a more meaningful relationship with Brugg than they do with any of the Duergar of the Hold.
Just to make absolutely sure that this isn't a satisfying showdown, it turns out Murkelmor is a runner. If he starts losing, he'll use a secret door to flee to the slave pits, and then double back looking for allies through the rest of the Hold. The dramatic clash of wills teased when the players first enter can devolve into a tedious running battle through a series of rooms they've already cleared.
 Forget about the secret door and forget about Murkelmor fleeing. There's nothing he'll find outside this chamber that'll be more satisfying for players than seeing him get beat down where he stands.
 Give Murkelmor the chance to show off either his strength or his villainy prior to the battle. Possibly have him visit the Seven-Pillared Hall and throw his weight around before players start the Hold. Maybe have him in the process of murdering one of the captives when the PCs burst in on him. At the very least have the other Duergar tell the players how scary he is. There's just no fun in defeating someone you've never heard of.