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  • Writer's picturetroodon135

Prehistoric Bestiary Pt. III: Prehistoric Sea Life

The prehistoric oceans were filled with all sorts of weird creatures that would fit in great in a D&D game. Most of the creatures included here are real animals, while others are scaled-up versions of creatures which in real life would be too small to be threatening.


Marine Reptile

While often mistaken for dinosaurs, marine reptiles are a diverse group of ocean-dwelling saurians. Some, such as the geosaurus and tylosaurus, are close relatives of modern reptiles, while others, such as the plesiosaurus, ichthyosaurus and kronosaurus, are truly alien to today’s oceans.


Geosaurus

These marine crocodiles are swift and deadly hunters which use their fishlike tails to propel themselves through the water.


Geosaurus

Large beast, unaligned

Armor Class 13 (natural armor)

Hit Points 22 (3d10+6)

Speed 0 ft., swim 50 ft.

STR DEX CON INT WIS CHA

14 (+2) 13 (+1) 14 (+2) 2 (-4) 10 (+0) 6 (-2)

Skills Perception +2

Senses passive Perception 12

Languages

Challenge 1/2

Hold Breath. The geosaurus can hold its breath for 1 hour.

 

Actions

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (1d10+2) piercing damage.


Ichthyosaurus

Ichythosaurs are strange, fishlike reptiles with enormous eyes and long, toothy snouts. The ichthyosaurus is the most common of these animals, a dolphin-sized creature that hunts squid and fish. Ichthyosaurus occasionally leap out of the water, sometimes to steal fish from unwary fishermen.


Ichthyosaurus

Medium beast, unaligned

Armor Class 13

Hit Points 11 (2d8+2)

Speed 0 ft., swim 60 ft.

STR DEX CON INT WIS CHA

10 (+0) 16 (+3) 12 (+1) 2 (-4) 12 (+1) 7 (-2)

Skills Acrobatics +5, Perception +3

Senses Darkvision 60 ft., passive Perception 13

Languages

Challenge 1/4


Hold Breath. The ichthyosaurus can hold its breath for 4 hours.

Keen Sight. The ichthyosaurus has advantage on all Perception checks that rely on sight.

 

Actions

Bite. Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 4 (1d8) piercing damage.


Kronosaurus

Like an enormous, mobile maw, a kronosaurus flies through the water on four wing-like flippers, eating anything that fits in its gaping jaws whether it be sharks, other marine reptiles, or even small boats.


Kronosaurus

Huge beast, unaligned

Armor Class 14 (natural armor)

Hit Points 93 (11d12+22)

Speed 0 ft., swim 50 ft.

STR DEX CON INT WIS CHA

19 (+4) 10 (+0) 15 (+2) 2 (-4) 10 (+0) 6 (-2)

Skills Perception +3

Senses Darkvision 30 ft., passive Perception 13

Languages

Challenge 6

Hold Breath. The kronosaurus can hold its breath for 2 hours.

 

Actions

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 43 (6d12+4) piercing damage and the target is grappled (Escape DC 15). If the target is Large or smaller, it is also restrained until this grapple ends. While grappling, the kronosaurus cannot bite any other target.

Swallow. The kronosaurus makes a bite attack against a Large or smaller target it is grappling. If the attack hits, the target is swallowed and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the kronosaurus, and it takes 22 (4d10) acid damage at the start of each of the kronosaurus's turns. A kronosaurus can have only one large creature, two medium creatures, or four small or tiny creatures swallowed at a time.

If the kronosaurus takes 20 damage or more on a single turn from the swallowed creature, the kronosaurus must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the kronosaurus. If the kronosaurus dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.


Tylosaurus

Tylosaurus are one of a diverse group of seafaring lizards called mosasaurs. These reptiles have long snouts filled with two sets of teeth, sharklike fins at the ends of their tails, and forked tongues like those of a snake. This particular species is distinguished by the bony ram at the end of its snout, which it uses to stun its prey.

Tylosaurus

Huge beast, unaligned

Armor Class 13 (natural armor)

Hit Points 76 (9d12+18)

Speed 0 ft., swim 60 ft.

STR DEX CON INT WIS CHA

18 (+4) 14 (+2) 15 (+2) 2 (-4) 10 (+0) 7 (-2)

Skills Perception +3

Senses passive Perception 13

Languages

Challenge 5


Stunning Ram. If the tylosaurus moves at least 20 ft. and then makes a ram attack on the same turn, the attack deals an additional 1d6 points of damage on a hit and the target must make a DC 15 Constitution saving throw or be stunned for 1 round.

 

Actions

Bite. Melee Weapon Attack: +1 to hit, reach 10 ft., one target. Hit: 26 (4d10+4) slashing damage. If the target is large or smaller, it is grappled (Escape DC 14). While the target is grappled, it is restrained, and the mosasaurus makes bite attacks against it with advantage, but it cannot make bite attacks against any other target.

Ram. Melee Weapon Attack: +1 to hit, reach 10 ft., one target. Hit: 21 (5d6+4) bludgeoning damage.


Primordial Sea Creatures

The most remote and primordial seas are inhabited by creatures even older than the dinosaurs. Some of these strange beasts resemble enormous versions of familiar animals, but others are entirely alien.


Cameroceras

Cameroceras resemble enormous octopuses with long, conical shells. While they are generally peaceful filter feeders, their tentacles can be powerful defensive weapons if they feel threatened.


Cameroceras

Large beast, unaligned

Armor Class 14 (natural armor)

Hit Points 37 (5d10+10)

Speed 0 ft., swim 30 ft.

STR DEX CON INT WIS CHA

14 (+2) 10 (+0) 14 (+2) 2 (-4) 10 (+0) 5 (-3)

Skills Perception +2

Senses Darkvision 60 ft., passive Perception 12

Languages

Challenge 1/2

Jet. When the cameroceras uses the disengage action, it moves at twice its speed.

Water Breathing. The cameroceras can only breathe water.

 

Actions

Tentacles. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6+2) bludgeoning damage. If the target is a creature, it is grappled (escape DC 12). Until this grapple ends, the creature is restrained, and the cameroceras cannot use its tentacles on another target.


Dunkleosteus

Dunkleosteus are among the largest of the placoderms, a diverse group of armored, prehistoric fish. Though the dunkleosteus is a deadly predator, it has no teeth, instead using jagged plates of bone to slice through prey.

Dunkleosteus

Huge beast, unaligned

Armor Class 15 (natural armor)

Hit Points 126 (11d12+55)

Speed 0 ft., swim 40 ft.

STR DEX CON INT WIS CHA

22 (+6) 8 (-1) 20 (+5) 1 (-5) 10 (+0) 5 (-3)

Skills Perception +3

Senses Darkvision 60 ft., passive Perception 13

Languages

Challenge 5

Armor Slicer. The dunkleosteus has advantage on attack rolls against foes wearing heavy armor.

Water Breathing. The dunkleosteus can only breathe water.

 

Actions

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 18 (2d12+5) slashing damage.


Giant Anomalocarid

Anomalocarids are the oldest apex predators. While most of these strange, lobed invertebrates are far smaller than a person, rare species grow to enormous sizes. The giant anomalocarid uses its prehensile, claw-like mouthparts to pull apart the armor of its prey, biting at the soft flesh beneath with its circular mouth.


Giant Anomalocarid

Large beast, unaligned

Armor Class 12

Hit Points 13 (2d10+2)

Speed 30 ft., type 30 ft.

STR DEX CON INT WIS CHA

14 (+2) 14 (+2) 12 (+1) 1 (-5) 10 (+0) 4 (-3)

Skills Perception +2

Senses passive Perception 12

Languages

Challenge 1/2


Keen Sight. The anomalocarid has advantage on all Perception rolls relying on sight.

Water Breathing. The anomalocarid can only breathe water.

 

Actions

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8+2) piercing damage. If the target is a Medium or smaller creature, it is grappled (escape DC 12). While grappled, the creature is restrained, and the anomalocarid cannot bite another target.

Peel Armor. The anomalocarid makes a bite attack against a grappled target. If it succeeds, the target must make a DC 12 Dexterity saving throw or its armor is peeled away, losing the AC it gains from armor or natural armor. A creature wearing armor can spend an action to readjust their armor to gain its AC again, while a creature with natural armor regains its natural armor AC the next time it regains hit points.


Giant Opabinia

Perhaps the strangest of undersea beasts, the opabinia has a segmented body, five compound eyes, and a grabbing claw on the end of a long proboscis which it uses to grab at prey. Regular opabinia are barely visible to the naked eye, but this enormous species is the size of a mid-sized dog. Fishermen in waters frequented by these creatures curse their ability to “snab” (possibly a portmanteau of “snatch” and “grab”) food from boats using their proboscis.

Giant Opabinia

Small beast, unaligned

Armor Class 13 (natural armor)

Hit Points 9 (2d6+2)

Speed 0 ft., swim 30 ft.

STR DEX CON INT WIS CHA

8 (-1) 15 (+2) 12 (+1) 1 (-5) 10 (+0) 4 (-3)

Skills Perception +2

Senses passive Perception 12

Languages

Challenge 1/4

Keen Sight. The opabinia has advantage on all Perception rolls relying on sight.

Water Breathing. The opabinia can only breathe water.

 

Actions

Claw. Melee Weapon Attack: +4 to hit, reach 10 ft., one target. Hit: 5 (1d6+2) piercing damage.



Giant Trilobite

The flat, segmented Trilobites were once the most common animals in the ocean, but in modern times they are only found in remote, isolated areas inhabited by other prehistoric beasts. Trilobites vary drastically in size, from smaller than a coin to the human-sized specimen presented here.


Giant Trilobite

Small beast, unaligned

Armor Class 15 (natural armor)

Hit Points 4 (1d6+1)

Speed 15 ft., swim 15 ft.

STR DEX CON INT WIS CHA

7 (-2) 10 (+0) 13 (+0) 1 (+0) 10 (+0) 5 (-3)

Senses passive Perception 10

Languages

Challenge 0

Water Breathing. The trilobite can only breathe in water


Helicoprion

Distant relatives to sharks and rays, Helicoprion are enormous deep-sea predators which slice prey apart using circular whorls of teeth on their lower jaws.


Helicoprion

Huge beast, unaligned

Armor Class 14 (natural armor)

Hit Points 76 (9d12+18)

Speed 0 ft., type 60 ft.

STR DEX CON INT WIS CHA

19 (+4) 14 (+2) 15 (+2) 1 (-5) 10 (+0) 5 (-3)

Skills Perception +2

Senses Blindsense 60 ft., passive Perception 12

Languages

Challenge 4


Keen Smell. The helicoprion has advantage on all Perception rolls relying on smell.

Water Breathing. The helicoprion can only breathe water.

 

Actions

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 23 (3d12+4) slashing damage. If the target is a living creature, it must make a DC 14 Constitution save or gain a bleeding injury. At the start of each of its turns, the creature takes 1d6 points of damage and must make a DC 14 Constitution saving throw. A success on this save ends the effect, as does any healing effect that restores hit points.


Jaekelopterus

Often referred to as “sea scorpions”, eurypterids like the jaekelopterus are only distantly related to true scorpions. Jaekelopterus is the largest of these underwater arthropods, growing as large as a crocodile. Jaekelopterus prefer brackish water where rivers meet the sea, snatching anything that comes within reach with their enormous, barbed claws.


Jaekelopterus

Large beast, unaligned

Armor Class 14 (natural armor)

Hit Points 10 (1d6+7)

Speed 20 ft., swim 40 ft.

STR DEX CON INT WIS CHA

15 (+0) 10 (+0) 16 (+0) 1 (-5) 10 (+0) 5 (-3)

Skills Stealth +2

Senses passive Perception 10

Languages

Challenge 2

Hold Breath. The jaekelopterus can hold its breath on land for 1 hour.

Water Breathing. The jaekelopterus can only breathe in water.

 

Actions

Claw. Melee Weapon Attack: +1 to hit, reach 5 ft., one target. Hit: 4 (1d6+1) slashing damage.

Tail Barb. Melee Weapon Attack: +1 to hit, reach 5 ft., one target. Hit: 4 (1d6+1) slashing damage.

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