Thalassic Behemoth, Scourge of the Abyssal Depths
“Unleash the terror of the Thalassic Behemoth: a colossal sea monster that devours ships whole and wreaks havoc on coastal towns!”

The Thalassic Behemoth is a staggering sight to behold, resembling a titanic sperm whale but with distinct, monstrous features. Its massive body stretches over 100 feet in length, covered in slick, dark-blue skin that shimmers with a bioluminescent glow in the depths of the ocean. Two powerful forelimbs, reminiscent of a seal’s flippers, drag its immense bulk across the sea surface and land. These limbs are muscular and equipped with sharp claws, capable of crushing wood and stone.
The behemoth’s head is broad and elongated, with a gaping maw lined with rows of razor-sharp teeth, large enough to swallow small ships whole. Its eyes, deep-set and glimmering like polished obsidian, can instill a primal fear in those who meet its gaze. A formidable tail, thick and powerful, serves as both a weapon and a means of propulsion in water, with the ability to create massive waves with a single strike.
Behavior
Thalassic Behemoths are solitary creatures, often lurking just beneath the ocean’s surface, using their exceptional swimming speed to outpace vessels in the water. They are not only destructive but also highly intelligent, exhibiting behaviors that suggest a strategic mindset. When attacking, they rely on a combination of brute strength and cunning, using their environment to their advantage. Thalassic Behemoths are known for their fierce territorial instincts; they will aggressively defend their domain from intruders, both aquatic and terrestrial. They communicate through deep, resonant calls that echo across the waters, instilling dread in sailors and coastal dwellers alike.
Habitat
These behemoths inhabit the vast expanses of the ocean, often dwelling in deep waters where they can easily hide from surface vessels. They prefer coastal regions, surfacing to wreak havoc on ships and settlements near the shore. Thalassic Behemoths are drawn to areas with rich marine life, feeding on schools of fish and smaller sea creatures. While they are primarily aquatic, they can drag themselves ashore for brief periods, though they quickly tire from the strain of gravity on their massive bodies.
Modus Operandi
The Thalassic Behemoth’s approach is typically heralded by unnatural phenomena: a sudden increase in missing ships, strange tides, and schools of fish beaching themselves in panic. When it targets a vessel, it utilizes its powerful swimming abilities to close the distance quickly. The behemoth may launch a surprise attack with its water jet, stunning sailors before attacking with its bite and claws. Once a ship is capsized, it may swallow smaller vessels whole or drag survivors to their doom with its mighty forelimbs. On land, it relies on its massive strength to demolish structures, creating chaos and destruction before retreating back to the water.
Motivation
Motivated by a divine mandate to punish those who dwell too close to the sea, the Thalassic Behemoth acts as an avenger of the ocean. Myths suggest that it was sent by ancient sea gods to restore balance and enforce respect for the aquatic realm. While its motivations may seem wrathful, they stem from a primal instinct to defend its territory and maintain the natural order. Legends say that the behemoth is not merely a creature of destruction but a guardian of the deep, ensuring that humanity remembers the perilous power of the sea. Its actions serve as a reminder of the ocean’s might and the consequences of hubris for those who seek to exploit its resources.
Thalassic Behemoth 5e
Thalassic Behemoth Pathfinder
Thalassic Behemoth

Gargantuan beast, neutral
Challenge Rating: 24 (62,000 XP)
Proficiency Bonus: +7
Attributes
- Armor Class: 23 (natural armor)
- Hit Points: 450 (30d20 + 150)
- Speed: 30 ft., swim 150 ft.
STR | DEX | CON | INT | WIS | CHA |
---|---|---|---|---|---|
30 (+10) | 8 (-1) | 25 (+7) | 3 (-4) | 16 (+3) | 14 (+2) |
Saving Throws
- Strength +19, Constitution +14, Wisdom +11, Charisma +9
- Damage Immunities: Poison, Psychic
- Condition Immunities: Charmed, Frightened, Paralyzed, Poisoned
- Senses: Blindsight 60 ft., Darkvision 120 ft., Passive Perception 19
- Languages: Aquan, understands Common but cannot speak
- Challenge Rating: 24 (62,000 XP)
Traits
- Amphibious: The Thalassic Behemoth can breathe both air and water.
- Legendary Resistance (3/Day): If the Thalassic Behemoth fails a saving throw, it can choose to succeed instead.
- Regeneration: The Thalassic Behemoth regains 20 hit points at the start of its turn. If it takes fire damage, this trait doesn’t function at the start of its next turn.
- Terrifying Presence: Creatures of the Behemoth’s choice within 120 feet that can see it must succeed on a DC 19 Wisdom saving throw or be frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Actions
- Multiattack: The Thalassic Behemoth makes three attacks: one with its bite and two with its claws.
- Bite: Melee Weapon Attack: +19 to hit, reach 15 ft., one target.
Hit: 4d12 + 10 piercing damage. If the target is a Medium or smaller creature, it must succeed on a DC 26 Strength saving throw or be swallowed whole. - Claw: Melee Weapon Attack: +19 to hit, reach 10 ft., one target.
Hit: 4d10 + 10 slashing damage. - Tail Swipe: Melee Weapon Attack: +19 to hit, reach 20 ft., all creatures in a 10-ft. wide line.
Hit: 4d12 + 10 bludgeoning damage and those hit must succeed on a DC 26 Strength saving throw or be knocked prone. - Water Jet (Recharge 5-6): The Thalassic Behemoth can unleash a powerful jet of water in a 60-foot line that is 10 feet wide. Each creature in that line must make a DC 26 Strength saving throw, taking 6d10 bludgeoning damage on a failed save, or half as much damage on a successful one. Additionally, on a failed save, a creature is pushed 20 feet away from the Thalassic Behemoth and knocked prone.
Legendary Actions
The Thalassic Behemoth can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Thalassic Behemoth regains spent legendary actions at the start of its turn.
- Detect: The Thalassic Behemoth makes a Wisdom (Perception) check.
- Tail Slap (Costs 2 Actions): The Thalassic Behemoth makes one tail swipe attack.
- Swallow (Costs 3 Actions): The Thalassic Behemoth attempts to swallow a creature it has grappled. The target must make a DC 26 Strength saving throw. On a failure, the creature is swallowed and takes 4d10 acid damage at the start of each of its turns. A swallowed creature is blinded and restrained. It can escape by using its action to make a successful DC 26 Strength check.
Lair Actions
On initiative count 20 (losing initiative ties), the Thalassic Behemoth can take one of the following lair actions:
- Tidal Surge: The behemoth causes a powerful wave to crash through its lair. Each creature within 60 feet of the behemoth must succeed on a DC 26 Strength saving throw or be knocked prone and pushed 15 feet away.
- Unnatural Fog: A thick fog rolls in from the ocean, heavily obscuring the area within a 120-foot radius of the behemoth’s lair. Creatures within this area must succeed on a DC 19 Wisdom saving throw or be blinded until the end of their next turn.
- Call of the Deep: The behemoth summons lesser sea creatures (such as sharks, giant octopuses, or merrow) to assist it in combat. The creatures appear in unoccupied spaces within 60 feet of the behemoth and act on the behemoth’s initiative count.
Regional Effects
The region containing a Thalassic Behemoth’s lair is warped by its presence, creating the following effects:
- Unnatural Tides: The tides within 1 mile of the lair are affected, causing sudden changes in water level that can capsize ships and leave them stranded.
- Marine Life Anomaly: Fish and other sea creatures within 1 mile of the lair behave erratically, often beaching themselves or fleeing the area.
- Terrifying Weather: Storms occur more frequently in the area, with violent winds and torrential rain during the behemoth’s rampages, creating treacherous sailing conditions.
Lore
The Thalassic Behemoth is a creature of myth, believed to have been sent by ancient sea gods as a guardian of the ocean’s depths. Legends tell of its ability to punish those who disrespect the waters, and its presence often foreshadows disasters at sea. Sailors speak of eerie calm before a storm and strange fish behavior that heralds the behemoth’s approach. Those who encounter it are faced with a choice: respect the ocean’s power or face the wrath of this titan of the sea.

Two massive limbs drag the bulk of this whale-like beast’s body from the churning waters of the sea.
[This content was created for the Pathfinder rules by Paizo Publishing LLC and is part of the Pathfinder RPG product line.]
Sent by the gods to punish those who dwell near the sea, thalassic behemoths scourge ships and other vessels from the waters before turning their attention toward structures and survivors on land. Despite their massive bulk, thalassic behemoths Swim incredibly fast and can easily outpace all but the fleetest of ships. On land, they can be more readily escaped, at least by those willing and able to leave homes and belongings behind. Though capable of surviving indefinitely on land and employing a clumsy, seal-like locomotion, thalassic behemoths soon weary of gravity’s never-ending drag. They rarely venture more than a mile or two from shore before returning to the comforting buoyancy of the seas.
Thalassic behemoths resemble immense sperm whales in the water, but they can quickly haul themselves ashore using their mighty forelimbs. Tail and limb alike shatter wood and stone with but a single blow, while their jaws gape wide enough to swallow smaller boats whole. The coming of a thalassic behemoth is typically foreshadowed by a dramatic increase in missing ships, strange tides, and the uncanny sight of sea life purposefully beaching upon shores. Thalassic behemoths generally spend much of their time lurking a few hundred feet below the surface’ but legends hold that even greater behemoths, the dread leviathans, dwell in the deepest oceanic reaches.
Thalassic Behemoth CR 20 |
XP 307,200 N Colossal magical beast (aquatic, behemoth, water) Init -1; Senses Blindsense 60 ft., Darkvision 60 ft..; Perception +34 |
DEFENSE |
AC 37, touch 1, flat-footed 37 (-1 Dexterity, +36 natural, -8 size) hp 391 (29d10+232); regeneration 20 (fire) Fort +24, Ref +17, Will +14 Defensive Abilities unstoppable; DR 15/epic; Immune ability damage, aging, bleed, cold, disease, energy drain, fire, mind-affecting effects, negative levels, Paralysis, permanent wounds, petrification, poison, polymorph; SR 31 Weaknesses vulnerable to miracles and wishes |
OFFENSE |
Speed 20 ft., Swim 200 ft. Melee bite +39 (6d6+18/19-20 plus grab), 2 claws +39 (3d6+18), tail slap +34 (4d6+9 plus trip) Ranged water jet +20 (4d8+18/19-20 plus stun and push) Space 30 ft.; Reach 20 ft. Spell-Like Abilities (CL 20th; Concentration +22) 3/day – tsunami (DC 21) Special Attacks capsize (DC 25), fast swallow, push (water jet, 20 ft.), rend (claws, 3d6+18), ruinous, swallow whole (4d6+27 bludgeoning, AC 28, 39 hp) |
STATISTICS |
Strength 47, Dexterity 8, Constitution 27, Intelligence 3, Wisdom 16, Charisma 14 Base Atk +29; CMB +55 (+59 grapple); CMD 64 Feats Ability Focus (water jet), Cleave, Great Cleave, Greater Sunder, Greater Vital Strike, Improved Critical (bite, water jet), Improved Iron Will, Improved Sunder, Improved Vital Strike, Iron Will, Lightning Reflexes, Power Attack, Skill Focus (Perception), Vital Strike Skills Perception +34, Stealth -13, Survival +18, Swim +30; Racial Modifiers +10 Perception SQ amphibious Language Aklo (cannot speak) |
SPECIAL ABILITIES |
Water Jet (Su) A thalassic behemoth can blast water from its blowhole with surprising force and accuracy. This water jet has a range of 240 feet with no range penalties. A target hit by the water jet is subject to the behemoth’s push ability and must succeed at a DC 32 Fortitude save or be stunned for 1d4+1 rounds. The save DC is Constitution-based. |
ECOLOGY |
Environment any water Organization solitary or pair Treasure none |
Section 15: Copyright Notice – Pathfinder Roleplaying Game Bestiary 3
Pathfinder Roleplaying Game Bestiary 3, © 2011, Paizo Publishing, LLC; Authors Jesse Benner, Jason Bulmahn, Adam Daigle, James Jacobs, Michael Kenway, Rob McCreary, Patrick Renie, Chris Sims, F. Wesley Schneider, James L. Sutter, and Russ Taylor, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.