Assuming someone checks the ghost's room before going, it held only a 4-foot tall bas-relief of a symbol of a large hand (which Jalon can identify as the symbol of Bane, the god of Tyranny) and a human-looking skeleton crumpled in a corner. Its skull and many other bones are utterly smashed.
After exploring this room, the party quickly explores the rest of the dungeon. At one point, a pair of ghouls attacked, but where quickly dispatched by the combined might of the adventurers.
(The finished dungeon map looks almost exactly like the one on page 127 of the Dungeon Master's Guide, by the way. Only difference is I knocked down a wall between rooms 24 and 39 to make room for the final battle with the priest.)
The dungeon is divided in two parts. The northern part, who was completely explored beforehand, and a southern part hidden by a not-very-secret door in the spider's room. Unlike the northern part, the southern part is fully illuminated by magical light still emanating from the ceiling after all these years. Its floor is also clean and not debris-strewn.
There are a couple of interesting areas in the southern half, as follows (I'm doing this because it'll take a month of real time easy to explore the rest of this level and it has no monsters or interesting traps except for the aforementioned pair of ghouls) :
- The hallway branching out near the corridor leading to the spider room (number 26) has a bas-relief on the northern wall showing corpses, and another one on the southern wall depicting skeletons. Under the bas-reliefs is an inscription :"That which is weak passes away. That which is strong endures." At the end of the hall are low stone tubs or vats and a chemical smell emanates from them. Quick experimentation revealed that they contained a very weak acid, now no more potent than, say, vinegar.
- Northwest from the ghoul's area is a winding corridor leading to a small room. This room contained undisturbed furniture: a cozy cot, a large chest, an enamel chamber pot, and empty weapon racks. It also held a pair of broken skeletons with the stingers and silvery runes. Searching the chest produced nothing of worth or importance, but the NE door held a secret door. The door led to two areas. One, to the east, held nothing interesting, but to the north, a stairway led down further into the darkness. Besides the stairway is a strange sight : a lit torch.
- To the southwest of the level lie a trio of rooms. The northernmost one (31) was a cell, with a strong door made to be barred from the outside and manacles chained to the walls inside. The door was not closed, however, and nothing interesting lay inside. The two others were bare, with bas-reliefs that depicted oxen, giants and monks performing feats of strength in the first room and in the other room, slave lords, conquering devils and a mighty monk standing over other monks in chains. The second room also had a skeleton in a corner.
- To the north of the room with the altar to Velsharoon where you fought the undead master, room 34 held tools and generic religious supplies, all very old.
- To the southeast was a small maze/catacombs with a couple of empty rooms. Room 35 had scenes of demons and devils as bas-relief and room 37 had a pair of old inanimate skeletons. The doors to both room 37 and 36 were bashed in. A secret door was also found leading to a small circular room (#38) containing an hexagonal altar unlike the one the undead priest was hiding behind. The altar was covered with dust and the sides of the altar were carved with shapes of daggers, one per side.
- Lastly, to the south of the room where the undead priest was is another set of stairs going down.
Here you go. If you have questions or things you want to do with any of those rooms, go ahead. You also have two sets of stairs going down to explore.