The Catacombs of Opposition (aka the Olthoi Town) is where a group of Isparians are holding out underground against a swarm of Olthoi brought on by the Ancient Olthoi Queen. In this dungeon you can find several Olthoi body parts and mundane generic items that can be given to the NPCs there in exchange for an Olthoi Weapon.
There are several different areas of this dungeon and a wide variety of Olthoi of different levels of difficulty ranging from Solidiers to Mutilators. Some of the Olthoi drop body parts used to make the weapons as indicated below.
The Catacombs of Opposition can be gotten to from any of the seven dungeons by first traveling through them and then through the Deeper Catacombs: