The Weeping Cistern
The floor here is a shallow bronze bowl forty feet across, and it is not dry. Water beads out of a seam in the far wall and runs down it in a sheet, steady as a held note, pooling around your ankles and finding the low ground. Three grates sit at the bottom of the bowl, green with age. Two of them are talking — a fat wet gurgle, water going somewhere.
The third makes no sound at all.
DM summary. A room that fills while the party works out which grate is a drain and which is a door. Nobody drowns here; the cistern's vault has an air gap by design, and the real currency is time, exhaustion, and how wet you are when you leave.
Terrain & Features
Battle map: The Basin Court — page 25, 55 × 75 ft (one half of The Temple Approach)
Use the sunken basin as the cistern's dished bowl: wall the court down to 40 × 40 ft around it, mark the three bronze grates at the low point, and hatch the weeping seam along the west wall. Wipe or ignore the outdoor dressing.
- Dimensions. 40 ft × 40 ft, ceiling 15 ft at the crown of a barrel vault. The floor is dished — the rim is level with the entry, the centre is 3 ft lower. The party comes in down five steps on the east side.
- Light. None of its own. Whatever the party brought, reflected off moving water onto the vault, which makes every surface look like it is crawling.
- The seam. A hairline crack the width of the west wall, weeping steadily. It has been weeping for a century; the wall below it is furred with lime an inch thick. Plugging it is not possible and a party that tries has spent a round.
- The three grates. Bronze, 3 ft square, at the bowl's low point, set north, south, and west. North and south are true drains, both half-clogged. West is not a drain. It is a maintenance plate over a dry crawl, and its bronze is worn smooth on the underside of the bars, where hands have pulled it up.
- The crown gap. Ten inches of air stay under the vault's keystone no matter how high the water goes — an overflow channel takes the rest. Say this to yourself before you run the room: this cistern cannot kill anyone by itself.
- The mooring ring. An iron ring in the north wall at chest height, lime- crusted, with four inches of rotten rope still through it. Somebody used to tie something up in here.
Traps / Puzzles / Checks
Trigger. The last person off the fifth step. A float-valve in the wall drops and the seam opens from a weep to a pour. The entry door swings to on its counterweight — not locked, just shut, and shutting is the whole trick.
| Element | Value |
|---|---|
| The clock | Water rises 2 ft per round, from ankle-deep. Waist by round 2, chest by round 3, over your head by round 5, at the crown gap by round 7 |
| Spotting the false grate | Perception DC 13 — two grates gurgle, one is silent. Investigation DC 14 on the grates themselves: no silt on the west one, and the wear is on the wrong side |
| Lifting the false grate | Thieves' Tools DC 15, or Athletics DC 17 bare-handed, or DC 13 with a crowbar, spear butt, or shield rim as a lever |
| Clearing a true drain | Reach in blind to the shoulder: Athletics DC 16. Success halves the rise rate to 1 ft/round. Failure: the clot — hair, wool, and a great deal of bone — shifts and grips. 2d10 bludgeoning and you are stuck for one round (Athletics DC 13 to pull free) |
| Forcing the entry door | Athletics DC 15, and the water goes out with you in a knee-high wave |
| Solved state | The west plate comes up, the water pours into the crawl behind it, and the room drains in three rounds. The crawl is exit C |
Treading at the crown. From round 7 the party is swimming with ten inches of air. Every 3 rounds spent there costs one level of exhaustion, no save. A character in heavy armour who does not shed it is at disadvantage on the Athletics checks to stay up, and dropping a breastplate into a cistern is a real decision, so make them make it.
Exits
None. A cistern with a working drain has been rinsing itself out for a hundred years; anything worth having went down the north grate a long time ago. The rope through the mooring ring is rotten and the ring is not coming out of the wall.
Scaling — APL 6 / 12 / 15
| APL | Adjustment |
|---|---|
| 6 | Water rises 1 ft per round and the entry door does not shut — the room is a puzzle with a cold, wet clock. All DCs −2. The drain clot does 2d10 and does not grip |
| 9 | As written |
| 12 | Water rises 3 ft per round and the overflow channel is blocked, so the crown gap is four inches, not ten. DCs +1. Drain clot 4d10 |
| 15 | As APL 12, plus the false grate is bolted from below (an extra Thieves' Tools DC 20 or 20 damage to the bolts) and the water is the maze's cold undercroft water: Constitution save DC 14 each round after the third or one level of exhaustion. DCs +3 |
- Pacing: 10–12 minutes. The room is a clock, so run it on the clock — call the water level out loud at the top of every round and never let a search take longer than it says on the tin.
- If you are behind schedule, start the water at chest height and already rising, and give the false grate away on a passive Perception of 13. Three rounds and a choice. Four minutes.
- If the party is thriving, let them find the overflow arch and take exit B on purpose. A party that decides to flood a room deliberately so it can swim out the top has understood the Labyrinth, and you should reward that with a clean, cinematic exit and no dice.
- Table safety. This is a drowning room, and drowning is a common line. Establish the crown gap out loud, in narration, before the water is at anyone's chin: "the vault is holding a band of air at the top, and it is not shrinking." If the imagery is still unwelcome, the seam can be sand instead of water with no mechanical change — sand rises, sand is heavy, sand does not drown you either.
- Clue drip. Two objects here carry a clue well: the mooring ring with its rotten rope, which is only there because something floated in this room once (X11, X16), and the lime line on the west wall, which is a tide mark and therefore a record of a tide (X16). If your Resolution is X14, the three grates are a bearing — the north drain runs downhill, and downhill is information.
- The room is not a monster. No wandering threat while the water is up. Whatever else lives near here does not want to be in a filling cistern either.