The Pitsudski Reservoir is a water cistern that is part of the pump system of the Cerro Rico Old Mines. A series of cistern chambers and water courses originally drained the water from the upper mines into the Pitsudski reservoir. The system of chambers functions as a huge hydraulic ram pump, drained the chamber through a dedicated water bore that ran parallel to the New Drift.
The ram pump failed when the disaster befell the mine and as a result the lower levels have been gradually flooding. Currently the lower level is waist deep in water.
This square chamber is partially flooded. It's difficult to tell how deep the blood red water that covers the bottom of the chamber is.
A narrow ledge around a foot above the water line circles the chamber to a closed door that is still above the waterline. The ledge is barely a foot wide, and seems to be covered with various kinds of colorful algae and lichen.
Floating corpses litter the surface of the lake. There are scores of them visible, most waterlogged and partially submerged..
The pool is 100 feet in diameter, the ledge is 1 feet high. The walls are completely smooth. The floor is also smooth. The current water level is at 6 feet. The roof is ten feet above the water line.
If more then three characters enter the pool, or more then five enter the ledge, they will activate the Zombie Swarm unless the party succeeds in a stealth check. Each round more zombies will rise from the pool to engulf them
The zombies individually are quite weak but there are hundreds of them, so directly navigating across the pool once they are active is not survivable.
When entering the pool area and for every round begun in the pool, there is a chance the pool will become activated.
Effects of being in the pool while activated
However the zombies are not fully mobile and cannot leave the pool. They can however attack people on the ledge.
After 10 rounds of activation and not having a target, the zombies will quiet down.
Methods of navigating the pool.
The Ledge:
The ledge is difficult to navigate and counts as difficult terrain. Acrobatics or Athletics rolls are required each round to maintain footing, DC12+. In the event that some falls, the characters near them may attempt a DEX check to save them. Difficulty is 12+ the amount the original roll was missed by. The character in front makes this roll with disadvantage.
Most of the algae on the ledge is harmless however at the first corner there is a patch of slime, Roll d6, 1-3 green slime, 4-6 yellow mold.
Stealthing across:
Characters can individually try to stealth across the pool or the ledge
Any attempt at stealth using bright light automatically fails.
Dim light requires Stealth 12 check every round spent in the pool or a DC8 from the ledge. If two characters are in the pool at once, each check is made at disadvantage. No light confers advantage on the stealth attempt but disadvantage on every other test.
Water Walking: across the pool gives advantage on stealth checks, negates difficult terrain, gives advantage on checks to avoid being grappled, removes the danger of being pulled under. However if more then 3 people disturb the pool, the stealth checks still fail. Due to the low ceiling, medium creatures cannot dash.
Flying: across the pool automatically fails stealth checks, negates difficult terrain, gives advantage on checks to avoid being grappled and allows a dex save for half damage. Due to the low ceiling, medium creatures cannot dash.
However there is a single Cloaker nesting in the rocks of the roof. It will automatically perceive any flyer and attack. However the Cloaker is well aware of the zombies in the pool and they are no more friendly to it then the party. If it captures a flyer it will attempt to abscond with them out the opposite entrance.
Elemental attacks on the pool
Electrocuting, freezing or boiling the pool are all options but will require the repeated application of powerful magic.
Attacks against the visible corpses may well destroy them, if the attacks are powerful enough, however there are many more corpses that are completely submerged and so such attacks will likely prove of little practical use.
Draining the pool: is not as crazy as it sounds. According the Stonebreakers map the chamber is located above the New Drift, on it's southwest corner. While there is 100 feet of rock inbetween, the rock is somewhat porous and unstable, and the water in the pool is immensely heavy.
Enforcer Montero used to delight in only one thing. Inflicting pain. A huge hulk of a man, he stalked the halls of Cerro Rico brandishing his long cat of nine tails, using it brutally on the miners, with little excuse required.
When the Dark Wind came. Montero was in the central chamber of The New Mine. The wind transformed him, giving him great strength and size, so much so that he was too large to walk the tunnels anymore.