Chapter 155 Schaub Empire Ruins
Cain steps through the portal, finding he is indeed underground, and there is none of the odd feeling that indicates he's in a dungeon instance. Two of the Oath Breakers guard this side of the portal, and he can see where they carried sand and rubble away from the Exit, moved here over time, something that doesn't happen inside a dungeon.
The others follow him through, leaving two Oath Breakers to stand guard outside and look around this underground space. They seem to be in the middle of an abandoned city, crumbling stone buildings, cobblestone streets, and some sort of bioluminescent fungus or moss in the roof, hundreds of meters above them that gives the whole area a light blue glow.