The cavern this last tunnel leads to is huge. With a torch there is no way to see all the way across the cavern. The walls are worked in areas of this almost round room. In other areas, it is just a natural cavern. The floor has mosaic tile here and there, but a lot of it's broken and no where near any other. There appears to be writing on the tiles, but in an ancient language and there are hardly two words strung together. (This should be some ancient language in your campaign with a hard DC and very rare that anyone knows it. This is a good place to fill in the history of the tunnels or to throw in a bone for your next adventure.) The walls have alcoves here and there, with reliefs of warriors and dragons and other mystical creatures carved right into the walls. The sounds of bats can be heard along with a raspy breathing, as if the whole cavern is breathing. There are racks of weapons and chests of equipment here and there.
This room was the sparing room and meeting room for an ancient sect of warriors who fought demons. In this room the party should find a demon appropriate for their level and his peons, (if appropriate for your campaign). There are also a lot of dead adventurers in this room, so this is a place for good treasure placement.