The Aethervale

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monster

The Sludge Behemoth

The Sludge Behemoth
Stat Block
CR 11
Reveal Stat Block
Armor Class
15
Hit Points
180
Speed
20 ft., swim 30 ft.
str
28
dex
8
con
26
int
4
wis
10
cha
1
str
28
+9
dex
8
-1
con
26
+8
int
4
-3
wis
10
+0
cha
1
-5

Senses blindsight 60 ft., passive Perception 10

Traits

Corrosive Form.A creature that touches the Behemoth or hits it with a melee attack while within 5 feet takes 4 (1d8) acid damage.

Actions

Multiattack.The Behemoth makes two slam attacks.
Slam.Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage plus 9 (2d8) acid damage.
Engulf.The Behemoth attempts to engulf a Large or smaller creature. The target must succeed on a DC 17 Dexterity saving throw or be engulfed. An engulfed creature is blinded, restrained, and takes 14 (4d6) acid damage at the start of each of the Behemoth's turns.

The Sludge Behemoth

Ecology

Born from a catastrophic Aether-pipe leak and the rampant dumping of toxic alchemical waste, the Sludge Behemoth is a terrifying amalgamation of corrupted magic and organic refuse. It is a mindless, insatiable force of destruction that resides in the deepest, most polluted sectors of the Aether-Sewers. It feeds on raw magic and biological matter alike, absorbing anything it touches to increase its mass. Its very presence poisons the water supply and causes the air to become highly corrosive.

Appearance

The Behemoth is a massive, shifting mountain of viscous, glowing green and black sludge. It has no fixed shape, constantly forming and reabsorbing thick pseudopods. Debris from the sewers—crushed barrels, rusted gears, and the skeletal remains of its previous victims—can be seen floating within its semi-translucent body. When it attacks, it forms crude, blunt appendages or gaping maws filled with jagged, half-dissolved metal scraps.

Lair Actions (The Flooded Aether-Sewer)

When fought in the central processing basin of the sewers, the Behemoth can alter the battlefield on initiative count 20:

  • Toxic Surge: The Behemoth causes the surrounding sludge pools to overflow. The entire floor becomes difficult terrain, and any creature starting its turn in the sludge takes 2d6 acid damage.
  • Aether-Venting: A pipe bursts, releasing a cloud of corrupted Aether-gas in a 20-foot radius. Creatures in the cloud must make a DC 15 Constitution save or become poisoned and unable to cast spells with verbal components due to coughing.
  • Assimilation Pull: The Behemoth exerts a magnetic, magical pull on nearby debris. All creatures must make a DC 15 Strength save or be pulled 15 feet toward the Behemoth.

Combat Mechanics

  • Corrosive Form: Any creature that touches the Behemoth or hits it with a melee attack while within 5 feet takes 1d8 acid damage. Non-magical weapons that strike it begin to dissolve, taking a permanent -1 penalty to damage rolls.
  • Engulf: The Behemoth can attempt to engulf up to two Large or smaller creatures. An engulfed creature is blinded, restrained, and unable to breathe, and takes 4d6 acid damage at the start of each of the Behemoth's turns.
  • Sludge Fling: As a ranged attack, the Behemoth can hurl globes of acidic sludge up to 60 feet, dealing acid damage and reducing the target's AC by 1 until the end of their next turn.
  • Mitosis (Reaction): If the Behemoth takes more than 30 slashing or lightning damage in a single turn, it instantly splits off a smaller, independent Sludge Spawn (use Black Pudding stats but scaled down) into an adjacent space.

Loot

Defeating the Behemoth leaves behind a massive, solidified core of purified Aether-slag. This Behemoth Core is highly prized by alchemists and artificers, and can be sold for a massive sum or used to power an airship engine for a decade. Scattered among its dissolved remains, players might also find an intact, acid-resistant magical item, such as a Ring of Acid Resistance or a pristine Aether-Valve Key required to seal the leak.