A gestalt entity composed of the consciousness of a number of the undead. Their bodies rot away but their minds remain and pool together by a shared rage and hatred of those still "alive".
There was more to this post but this portion is along the lines of what I was thinking when reading it.
The brainless undead: for some reason I pictured a big crater that these zombies would, at some point, accidently fall into and not be able to get out of. Could be a herding point when they become unmanageable as well. I think the undead with intelligence may get annoyed with the crowds of wandering useless corpses that would start to infest the place over time. So they simply herd a number of them into one of the many pits around the city. Might be the same place that dead undead (?) get thrown as well. Undead aren't sanitary but it could be the rotting corpses laying around would get annoying enough to want to move. On this tangent there probably are bodies laying all over the place.
Anyway, my thought was that it would be a pit of agony, moaning, clawing to escape undead that don't have enough brains to climb, but enough to claw the walls. This clawing may even form other caves and such, but anyway, just thinking of a place to put all these undead that are utterly useless to the undead society.
But what if, brainstorming off CirrusWinds idea, this groaning mass did rot and start combining and mixing in such a way that the undead intelligence (what little there was) would combine into a giant sliming blob that could escape the confines of their pits. The mindlessness and because it is a blob would make it difficult to destroy, could ingest anything in its path and absorb it, perhaps even gaining the intelligence of those it consumes and combining it. So smart people would only insignificantly add to its intelligence but add it would.