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Vitriolic Behemoth - AI-generated fantasy Monster

Vitriolic Behemoth

The creature was once Kaelen, a brilliant but overworked assistant to the rogue alchemist Elara Vane. During an experiment to create a universal solvent, a vat ruptured. Kaelen pushed Elara to safety but was submerged in the unstable mixture. The acid fused with his bone marrow, turning his skeleton to obsidian and his flesh into a living, melting catalyst.

Monsters
Large aberration · Hard · Unique

Vitriolic Behemoth

A tragic lab assistant transformed into a massive, translucent acid-slug with a visible obsidian skeleton.
AC15
HP126/126
CRHard
Init
STR
DEX
CON
INT
WIS
CHA
ConditionsNone

Combat

AC 15 · HP 126 · Hard

Vitriolic Behemoth

Large aberration (mutant), neutral evil
Armor Class15 (Natural Obsidian Growth)
Hit Points126 (12d10 + 60)
Speed
20 ft.,
Climb 20 ft.
STR
DEX
CON
INT
WIS
CHA
Saving ThrowsStrengthConstitution
SkillsAthleticsPerception
Damage Resistances
Bludgeoning, Piercing, and Slashing from nonmagical attacks
Immunities
Acid,
Poison
Weaknesses
Cold (Slows movement by 10 ft. until end of next turn)
Senses
Blindsight 60 ft.,
Passive Perception 13
Languages
Common (Broken, gurgled phrases)
ChallengeHard
Proficiency Bonus+3
Corrosive Skin. Any creature that touches the Behemoth or hits it with a melee attack while within 5 feet of it takes 5 () acid damage. Any nonmagical weapon made of metal or wood that hits the Behemoth corrodes; after dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed.
Amorphous. The Behemoth can move through a space as narrow as 1 inch wide without squeezing.
Spider Climb. The Behemoth can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Actions

Multiattack. The Behemoth makes two Pseudo-pod attacks. It can replace one Pseudo-pod attack with a Gurgling Grab.
Pseudo-pod. Melee Weapon Attack: to hit, reach 10 ft., one target. Hit: 12 () bludgeoning damage plus 7 () acid damage.
Gurgling Grab. The Behemoth attempts to grapple a Medium or smaller creature within 10 feet (Escape ). While grappled, the target is restrained and takes 10 () acid damage at the start of each of its turns.
Corrosive Breath (Recharge 5-6). The Behemoth exhales a spray of vitriol in a 30-foot line that is 5 feet wide. Each creature in that line must make a Dexterity saving throw, taking 35 () acid damage on a failed save, or half as much on a successful one. The area of the line becomes an Acid Puddle until the end of the Behemoth's next turn. Any creature that enters or starts its turn in a puddle takes 10 () acid damage.

Reactions

Splatter. When the Behemoth takes more than 15 damage from a single piercing or slashing attack, a burst of acid sprays from the wound. All creatures within 5 feet must succeed on a Dexterity saving throw or take 7 () acid damage.
Role
Environmental Controller / Brute
Built for
6

Gear

  • Melted Locket (Curio, 25 gp)A silver locket partially fused into the Behemoth's glass-like ribs. It contains a smeared portrait of Elara Vane.

Notes for the game master

Highly resistant to mundane physical attacks.
Acid Breath should be used to cut off player retreat.
Grappling is the priority over raw damage.

Tactics

  1. Opening

    The Behemoth uses Corrosive Breath immediately to coat the most likely escape route in acid, trapping the party in the room.

  2. Mid-fight

    It focuses on grappling the closest target and dragging them into the acid puddles it created. It uses its climb speed to stay on walls, striking from above with Pseudo-pods.

  3. End of the fight

    As it nears death, it gurgles 'Kaelen... is... sorry...' and focuses all attacks on the creature that has dealt the most damage to its obsidian skeleton.

  4. Retreat

    It does not flee. It is too far gone. If reduced to 10 HP, it attempts to merge into the acid puddles on the floor to hide, though it remains hostile.

How it fights

Always pull grappled targets into hazards.
Target characters wearing metal armor to trigger the corrosion effect.
Position in narrow corridors where its 30ft line breath weapon is unavoidable.

Who it attacks first

Characters dealing Cold damage (highest priority to remove its weakness)
Clerics or Paladins (perceived as Elara's rescuers or rivals)
The nearest physical attacker

Works well with

Gray Oozes (the Behemoth is immune to their acid and vice-versa)
Rust Monsters (corrosion focus)
Difficult terrain like slippery slime or toxic gas

How to beat it

Ray of Frost or other cold damage to slow it down.
Forced movement to keep it away from puddles.
Magical ranged attacks that bypass Corrosive Skin.

Ways to avoid a fight

Presenting Elara Vane's journal may cause the Behemoth to hesitate for 1 round.
Casting 'Remove Curse' or 'Greater Restoration' doesn't revert the form but might silence the gurgling and cause it to stop attacking for 1d4 rounds out of confusion.

Ecology

A massive, multi-limbed slug with translucent, jelly-like flesh. Its internal organs are barely visible, but its skeletal structure, made of jagged, black obsidian-glass, shines through the goo. Six vestigial, spindly human-like arms twitch near its front, while two massive pseudo-pods serve as its primary limbs.

Behavior

It moves with a ponderous, wet suction sound. It is driven by a constant, burning hunger and a fading memory of its former humanity, causing it to gurgle distorted pleas in Common that sound like 'Help... cold... burning...'

How it lives

The Behemoth is a biological anomaly. It does not eat in the traditional sense; it dissolves organic matter and absorbs the nutrient-rich slurry through its skin. It leaves a trail of smoking, pitted stone wherever it slides.

Lair

Usually an old laboratory basement floor covered in 3-inch deep caustic fluids and piles of dissolved organic matter.

Habitat
Dark dungeons, ruined alchemical laboratories, and sewers.

Signs that it is near

Pitted stone flooring with deep etchings
A pervasive smell of vinegar and scorched copper
Gurgling sounds that mimic weeping
Dissolved equipment and metal scraps

Tags

Acid,
Grappler,
Dungeon-Dweller

Lore

The creature was once Kaelen, a brilliant but overworked assistant to the rogue alchemist Elara Vane. During an experiment to create a universal solvent, a vat ruptured. Kaelen pushed Elara to safety but was submerged in the unstable mixture. The acid fused with his bone marrow, turning his skeleton to obsidian and his flesh into a living, melting catalyst.

What drives it

A desperate search for a cure that no longer exists and a primal urge to soothe its internal chemical burns with fresh meat.

After the fight

Once defeated, the Behemoth's flesh dissolves rapidly into a harmless puddle of water, leaving behind only the jagged, brittle obsidian bones and any gear it had absorbed. The sorrowful gurgling stops, leaving an eerie silence in the dungeon.

“During an experiment to create a universal solvent, a vat ruptured.”
From its lore

Variants

Crystalline Behemoth: Swaps Acid for Necrotic damage, skeletal system emits a dim, violet light.
Laboratory Guardian: Has 18 AC and is fused with metal plating, reducing movement but increasing durability.

Hooks

Plot hooks

Elara Vane hires the party to 'rescue' her former assistant from the ruins of her old lab.
A local water supply is turning acidic, traced back to the Behemoth's new nest.
A scholar seeks a sample of the 'Living Obsidian' skeleton inside the beast.

How to use it

Final boss of a corrupted laboratory crawl.
A trap-turned-beast in a flooded sewer network.
An environmental hazard during a chase scene.

Treasure

150 gp worth of raw Obsidian Shards (harvested from corpse)
Ring of Acid Resistance (fused into its mass)
Vial of Potent Acid (can be used as 3 doses of Alchemist's Fire, but deals acid damage)

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