Note: This information was taken from TSR's original AD&D adventure, the Temple of Elemental Evil.
For the players: This oddly shaped chamber is bisected
by a curving wall of well-dressed, mortared
stones. The wall is about four feet
tall, and to the south side of it is water,
high enough to almost flow over the top.
Water trickles in from several places
along the east, south, and west walls;
overflow gravitates to floor drains even
as you watch.
For the DM: Careful examination of the walls and
floor within the cistern discovers that the
stones are covered with some form of algae,
apparently quite slimy and presumably slippery.
The floor of the cistern immediately
south of the retaining wall is four feet lower
than that in the room, and slopes down to
the south about two feet for every five;
thus, at the far south end, the cistern is
about 15 feet deep. The overflow drains
carry the water to the dungeon level below.
Anyone trying to stand on the slippery
cistern floor slips on the slimy growth (no
saving throw) and slides to the lowest part
of the cistern, drowning in 3-5 rounds unless
rescued or light enough to float. In that
deepest portion is a large round slimecovered
object, which is a shield +2.
