There's not much to say about this encounter so I'll keep it brief.
The Hall of Howling Pillars is a straightforward ambush. Players need to get their hands on the bell of fury's calling, located on an altar in the southeast, but they're obstructed by (a) the "howling pillars", which are horrific columns of living, tortured flesh that scream (and vomit) at adventurers who disturb their domain, and (b) a pack of carnage demons hidden with the pillars.
When the PCs get near the altar, the carnage demons emerge from their hiding spots and a battle ensues.
I actually ran this one wrong; I had the demons emerge too early, allowing the PCs to pick them off in pairs, so I've got no real feel for how it works when used as intended.
Probably the most interesting aspect, mechanically speaking, of this encounter is the bell itself. All of the items hidden in the rooms around the Proving Grounds are in some way cursed, and the curses tie into the themes of rage and madness that underlie the history of Saruun Khel. It's interesting in that cursed magical items, although part of a long tradition of D&D, have been removed from 4th Edition, and to some extent Thunderspire is going to lengths here to re-introduce them.
The intention of the 4th Edition rules is that magical items are always beneficial. Their usefulness varies, but you're never worse off as a result of finding them. And so the cursed items in Thunderspire follow that idea. Each of the three items has a horrible drawback, but it's balanced out by a compensating benefit.
For example, the bell of fury's calling has a handle covered with terrible spikes. Ringing it causes the wielder to take 2d10 damage. Everyone who hears the sound goes into a mad frenzy, causing their defences to lower but their attack and damage rolls to enjoy a healthy +2 bonus. It's a vicious item and players have to think carefully about the trade-offs involved.
I actually really enjoy this approach to magic item design. These kind of "cursed items" really convey a sense of the risks involved in dabbling with magic, and I'd love to see sacrifice/buff items explored further in Adventurer's Vault supplements. Like all min/maxing items there's very real potential balance issues involved but judging by what we've seen so far of the Adventurer's Vault 2, item balance is clearly not something that troubles the developers, so we can at least hope for broken items that are cool.