The only real threat we encountered was a flameskull, a kind of animated floaty magic burning skull that typically acts as a guardian in ancient crypts, magical laboratories, and the like. It hit us with a fireball, which hurt, but we quickly defeated it... only to discover that the thing was reforming as we examined the room. Not knowing the methods of permanently dispatching it (Obsidian could have done the job with dispel magic had she known), the party fled past it instead, finding the missing smuggler trapped in the room beyond, unable to get out through the constantly-reforming undead thingie.
We could have probably gone out there and fought the thing again, but we were getting low on spells (laurie_robey's cleric having to use much of her healing ability on our intended rescue targets), so Obsidian decided it was time for a little finesse. She cast major image to create an illusion of hantamouse's fighter/rogue and had it run out into the room and act as a giant target, backed up by a spiritual weapon actually engaging the flameskull. While the undead guardian turned and flung all its spells at the illusion, the party snuck out of the room behind it.
Obsidian would like to note, for the record, that drow do NOT go "Tee hee." But as her player, I suspect she may be lying about that.
We did find some other things relevant to the larger campaign metaplot as well, and we are now organizing a return trip down into the place to follow up on that when we don't have to shepherd any injured NPCs. This, however, will have to be done by stealth, as the mercenary guild is stubbornly refraining from actually "hiring us" do go back down there, as Obsidian would have preferred.
My own campaign will probably continue next week, and by coincidence, the party in that game are going to be poking around a recently-unearthed ruin that may or may not have traps and flameskulls in it. Just sayin'. I think that I'm going to stick with the house rule of having players roll Perception to spot traps, rather than going with Passive Perception, because since I set the DC to spot the trap, using P.P. basically means that I just decide up front whether they spot the trap or not. (See Item 17 of My Gamemastering Credo.) This was particularly noticeable in tonight's complex full of traps, as we pretty much spotted all but one of them well in advance.