The big room with the hole in the center and the strange title was a temple with a priest, a big town will almost always have two temples: one sacked, one active.
One of them will have four doorways and tunnels leading off of the lower levels, the other will be flat on the surface or a pit with no tunnels generally. SOMETIMES you can find both temples with tunnels, but don't count on it, I've never found a town with two active temples but it might happen.
When you're down inside the catacombs/dungeons under a temple/keep and find someone, talk to them and ask them about the structure/dungeon/sewer/catacomb/etc and if they mention the group you're after, they are in it.
You can tell stories about the site to learn when The Groupname of Something moved into Something the Dungeon of Sewerness or whatever and they'll have a mention of becoming the primary criminal organization in that town. When you're actually in those dungeons/sewers/catacombs you see the right name down at the bottom left, Stinkfilths the Plague of Reckoning or whatever.
If you check under the keep and temples and still can't find them, travel around with the right map in the names mode and look for a purple Sewer, that'll have a stair or ramp down into those and you should be able to track them down in there, plus find some possible loot on the bodies or get jumped by a bunch of fish people!
Stupendous and unheard of splendors await you below, and you shall seek them soon!