Catacombs of the Lords of the End | Dungeon 23

After the intrepid explorers find the formerly sealed secret door they descend down into a small chamber using a ladder. That chamber is connected to a catacomb with 36 remaining alcoves for remains, four small doors to private burial chambers and a double door.

The walls weep. Now somewhat below the waterline of the bog-city of Sheljar there is a heavy amount of moisture, plus the southwest corner is open to the brackish waters of the area. Outside of the open pool are a large pile of bones. It is clear that the opening is natural, not manmade.

Possible learnings

Intelligence checks attempting to understand what could make such a large opening could use Nature, History, Culture (Sheljar) to understand that damage was done by a Tunneling Nightmare (DC 15).

Intelligence checks attempting to understand the alcoves and small doors could use Religion, Culture (Sheljar) to understand that the small doors are for former leadership of Sheljar (DC 10).

Inspecting the bones using Survival or Perception would find that they are recent to the area, not covered in dust (DC 10) and there are now 4 skeletons attacking the group.

Complications

If 50% of the group takes enough damage to fall unconscious making death saves the roar of a Tunneling Nightmare echoes, partially muffled by the water.