The Blackwater Spider Cistern
The Spider-Filled Cistern is an environmental hazard in which web-choked stonework, concealed spider swarms, and fouled water turn a seemingly ordinary dungeon reservoir into a living ambush site. The danger comes from movement, noise, and the characters' own attempts to cross the room.
The Blackwater Spider Cistern
Defenses
Detection
“The cistern is a round, sunken chamber filled with ankle-deep black water and dense curtains of webbing. Pale spiders cling to the walls, the ceiling, and the undersides of loose stones. A cracked stone drain descends into deeper darkness at the center of the room.”
Standard Engagement
The hazard triggers when a creature enters the central pool, touches the central drain, tears down a major curtain of webbing, attacks the Brood Nest, or makes a loud impact in the chamber. It also triggers if three or more creatures enter the cistern at once.
On initiative count 20, if the hazard has been triggered, the cistern produces one effect determined by the creatures' positions: creatures in web-filled areas are targeted by Clinging Webs, creatures in the central pool risk the Suctioning Drain, and creatures near the walls are attacked by skittering spiders. The DM can describe this as movement and pressure rather than a separate monster attack. The hazard does not make attack rolls.
After the initial eruption, the remaining spiders retreat into cracks and resume waiting. The hazard resets after 1 hour unless the Brood Nest is destroyed. If the nest survives, the hazard can trigger again whenever a creature crosses the central pool or tears down a major web curtain.
Each creature that starts its turn in a web-filled area must succeed on a DC 13 Dexterity saving throw or become restrained by sticky webbing until the end of its next turn. A restrained creature can use its action to make a DC 13 Strength check, escaping on a success. The webbing can also be destroyed as described under Countermeasures.
When the hazard triggers, a cloud of irritated spiders erupts from the walls and water. Each creature in the cistern must make a DC 13 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one. A creature that fails the save is poisoned until the end of its next turn.
A creature that moves through the central pool or falls into the drain must succeed on a DC 13 Strength or Dexterity saving throw or be pulled 10 feet toward the drain and knocked prone. The creature takes 7 (2d6) bludgeoning damage and must repeat the save at the start of its next turn if it remains in the drain's current.
The cistern is dimly lit only where characters provide light. Web-filled squares are difficult terrain. A creature that dashes through a web-filled square has disadvantage on the Dexterity saving throw against Clinging Webs.
The spiders are highly sensitive to vibration. If a creature uses thunder damage, falls, or makes a loud impact in the chamber, the hazard triggers immediately and all creatures have disadvantage on their first saving throw against Venomous Swarm.
The Brood Nest is hidden beneath a loose central stone. It becomes visible when the stone is moved, when the chamber is brightly illuminated, or when a character succeeds on the DC 15 Wisdom (Perception) check described under Discovery.
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