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Image promptamphibious aberration lurking beneath a still black pool, eyeless lamprey maw with hooked piercing teeth, slick grey-green rubbery ribbed flesh, half-submerged motionless ambush predator, dragging tendrils, flooded dungeon lower level, wet masonry, murky reflective water, single distant torch reflection, tight low tense composition, grey-green and black palette, dark fantasy, grimdark, painterly digital art, dramatic chiaroscuro lighting, muted desaturated palette with corrupted-crimson accents, weathered post-Cataclysm atmosphere, intricate high detail, cinematic composition

Cistern Lurker

Medium Aberration, Unaligned
CR 2LurkerUnderground
AC 13
HP 39 (6d8 + 12)
Speed 20 ft., Swim 40 ft.
STR
15
+2
save +2
DEX
16
+3
save +5
CON
14
+2
save +2
INT
6
−2
save −2
WIS
13
+1
save +1
CHA
5
−3
save −3
Skills Stealth +7 (in water +9)
Senses Darkvision 60 ft., Tremorsense 30 ft.; Passive Perception 11
Languages —

Traits

Amphibious. Breathes both water and air.

Still Waters. While the Cistern Lurker is in water without moving, noticing it requires a DC 16 Wisdom (Perception) check — DC 18 if the water is murky or dark.

Ambush Predator. The Cistern Lurker's first attack made from the Hidden condition deals an extra 7 (2d6) Piercing damage.

Actions

Drag Under. Melee Attack Roll: +5, reach 10 ft. Hit: 8 (1d10 + 3) Piercing damage, and if the target is Large or smaller it makes a DC 13 Strength save. Failure: The target is Grappled (escape DC 13) and pulled 10 feet toward the Cistern Lurker.

If the Grappled target is pulled into the water, it cannot breathe (Suffocation rules).

Bonus Actions

Submerge. The Cistern Lurker enters the water and takes the Hide action. It needs no place to hide while doing so underwater — the water is enough.

How to Use

Scene: A still pool on the lower level of the dungeon. Knee-deep water. The party has to cross.

Loop:

  1. The party enters the water. Nobody rolls (Passive Perception is usually 12-14, doesn't meet DC 16).
  2. The Cistern Lurker strikes from ambush: 8 + 7 = 15 damage + Grappled + 10-foot pull.
  3. It drags the victim underwater. They can no longer breathe.
  4. The party comes to rescue them → the Lurker vanishes with Submerge.

The real threat isn't the damage, it's drowning. Rescuing the Grappled character requires someone else to enter the water — and the Lurker is waiting there for them.

How the party wins: Rolling Perception before entering the water, or draining/freezing the water (Ray of Frost, Create or Destroy Water), or moving slowly to fool the Tremorsense.