Rushing water is as deadly as fire, and can carry away the largest ogre in a moment. The Passage of Devouring Waters is an elaborate defense that uses the power of water to augment a larger structure with a deadly surprise. If things go as planned, your party may never crawl into a villain’s lair through the sewers again.

As presented here, the trap can be within a city or carved from deep mountain tunnels, perhaps to guard the secret entrance of a temple complex. Click the map for a larger version with full detail.

1. Temple Baths – Fed by aqueducts, these cisterns hold the water supply that powers the trap; the chambers contains sumptuous baths for cleansing rituals. One bathhouse is for the general occupants and the other is a private bath for the high priesthood (which is filled with divinely-created water rather than aqueduct water). Small drains in the floors of both baths are nonfunctional feeding shafts for the trap below. What do they feed?

2. Entry Guard Room — Visitors must climb a 80′ ladder to reach this room; at the top, a heavy stone grate seals the tunnel. The guards keep a barrel of oil near the grate. In case of an alarm, they dump the oil down the grate, coating area 6 in oil and making the ladder slick and dangerous.

3. Green Slime Holding Chamber – The only entry to this room is from #2, down a short stone stair to a platform overlooking an enormous colony of green slime. The temple servants toss their refuse into the green slime help keep the complex hidden from creatures on the surface, and the priests regularly kill portions of the green slime, when it outgrows the pool. Stone plugs attached to a stone chain that drops down into area 4 keep the feeder shafts clear.

4. Drain Tunnel – This tunnel connects areas 3 and 5. At the end of this shaft is a stone plug holding up the excess chain connected to the stone plugs in area 3.

5. Entrance Tunnel — This single route into the complex is five feet wide, five feet tall and 160 feet long from the bottom of the ramp to the base of the guard tunnel and entry to the temple (area 6). Groundwater seeps in when it rains, and two to three inches of standing water cover the dirt floor. The passage is deliberately kept dirty, so that when the passage partially floods, the mucky water provides little visibility.

6. Entry Shaft – Visitors must ascend this shaft to enter the complex. A ladder is set into the wall. When oil-slicked, this is a DC 40 Climb check.

How The Composite Trap Works

Three traps defend the passage, one as a trigger and two that follow closely on its heels.

The Setup Goes Boom: A glyph of warding is set in the floor in area 5. If someone passes within 5 feet of this glyph without saying the passphrase—a religious saying of the cult—the glyph explodes. The DC for the glyph is higher due to the standing water in area 5.

The explosion destroys the supports maintaining the stone plug at the bottom of area 4. When this plug drops, it pulls the chain and the two stone plugs in area 3 are released. The high-pressure flow of water from area 1 washes the green slime from area 3, and the outflow from area 4 pulls the slime into the flooding tunnel of area 5. After three rounds, the tunnel is filled (yes, we did the math for it).

Three Rounds of Slime: During those three rounds, the green slime suspended within the rushing water may coat any unfortunates still within the tunnel. In the round the glyph goes off, the slime makes a touch attack against anyone in the cross-shaped junction of area 5. In the next round, it makes a touch attack against anyone in the first straight sections beyond the junction. In both the next round and the final round the passage fills, the slime makes a touch attack against anyone in area 5.

Anyone still in area 5 when it is filled must either hold their breath or be able to breath water—although anyone breathing the water takes a –2 circumstance penalty to their touch AC versus the slime attack. The water is cloudy and provides concealment beyond 5 feet. Characters can hold their breath for a number of rounds equal to twice their Constitution score before they begin drowning (DMG, page 304).

The Slime Itself: Green slime doesn’t require air to attack, and does Constitution damage until it is cleaned off (DMG, page 76). A cure disease spell cast before the tunnel fills completely resets the slime’s attack sequence to the round the glyph activates. If cast when water fills the tunnel, the spell destroys all green slime in area 5, but not beyond area 5. Some more ruthless DMs may consider the green slime in the water and the green slime consuming characters to be different patches, each requiring a spell to cleanse.

Guards monitor the oil-slicked ladder in area 6, possibly setting the fuel ablaze to kill survivors who attempt to climb up into the temple.

Afterwards, the priests allow the water to seep out naturally, cast stoneskin on a priest who descends the shaft and casts cure disease until the tunnel is clean. That priest resets the plugs in areas 3 and 4. Area 3 is rarely scrubbed clean of slime, but if it is, a priest collects some from area 5 to restart the colony before destroying the rest.

This trap seems expensive, but judicious use of slave labor, later fed to the growing slime colony, and stone shape spells can mitigate much of the cost.

The Passage of Devouring Waters, CR 12
Glyph of Warding: CR 5; spell; spell trigger; no reset; spell effect (glyph of warding [blast], 5th-level cleric, 2d8 acid, DC 14 Reflex save half damage); multiple targets (all targets within 5 ft); Search DC 30; Disable Device DC 28. Cost: 350 gp.

Flooding Corridor: CR 11; mechanical; mechanical trigger; automatic reset; multiple targets (all targets in corridor, 32 squares.); never miss; onset delay (3 rounds); liquid; Search DC 17; Disable Device DC 23; Cost: 89,600 gp.

Slime Gout: CR 6; mechanical; proximity trigger; manual reset; area effect (Special, see description); Atk +6 touch; 1d6 Con damage/round until removed; Spot DC 15; Disable Device: Special. Cost: 450 gp (stoneskin spell needed to collect the slime).

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