Uneasy Rests the Crown'd Head (5E)

From publisher blurb: A 5th Edition Mini-Dungeon for 4-6 PCs of levels 7-8 A sinkhole, cut with climb-holds, descends a cramped cavern’s south wall, slick with more than water. A 3-ft.-wide path rings a black, light-absorbing lake. 20-ft. north of the path, a stone circle containing a clear membrane floats above the water. A path leaves the cavern in the north wall, a crack in the ceiling drips water. The lake houses an aboleth sanctuary, the start of a new city.