It's said that the Swordlords sent four expeditions into the Stolen Lands. The PC's expedition, Maegar Varn's and Baron Drelev's are three. What's the fourth?
The Iron Wraiths, into Pitax. Basically Irovetti smashes them so fast nobody ever hears from them again -- though I've seen some people suggest that the ranger encounter in Book Five might be the last surviving Iron Wraith.
The Iron Wraiths, sent to the Glenebon Uplands to deal with the barbarians and any border issues with Pitax. Seeing as they don't seem to be heard from or mentioned again after the sidebar note about them on page 13 of "Stolen Lands", I'm guessing they failed in their mission rather early on.
There is a forum labled iron wraiths on here that give suggestions on alternate ideas for them.
The official line (although I can't link to the thread I saw it in) is that the Iron Wraiths were killed / defeated by the barbarians in the Glenbon uplands/heights.
Which is, perhaps, one of the reasons Fort Drelev had so much trouble developing into a successful city state.