Gargantuan monstrosity, unaligned

Armor Class 25 (natural armor)
Hit Points 410 (20d20 + 100)
Speed 0 ft., swim 20 ft.

30 (+10) 10 (+0) 30 (+10) 2 (-4) 8 (-1) 8 (-1)

Skills Perception +7
Saving Throws Str +26, Con +26
Damage Vulnerabilities fire
Damage Resistances cold, lightning
Damage Immunities poison; bludgeoning, piercing, and slashing from nonmagical attacks
Condition Immunities exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, stunned
Senses tremorsense 120 ft., passive Perception 17
Proficiency Bonus +8
Challenge 27 (105,000 XP)


  • Ambusher. In the first round of a combat, the jasconius has advantage on attack rolls against any creature it surprised.
  • Barbed Hide. At the start of each of its turns, the jasconius deals 5 (1d12) piercing damage to any Gargantuan creature grappling it.
  • False Appearance. While the jasconius remains motionless, it is indistinguishable from an inanimate island.
  • Siege Monster. The jasconius deals double damage to objects and structures (included in Whirlpool).
  • Water Breathing. The jasconius can breathe only underwater.


  • Multiattack. The jasconius makes two attacks: one with its Bite and one with its Tail.
  • Bite. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 28 (4d8 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 26). Until this grapple ends, the target is restrained, and the jasconius can’t bite another target.
  • Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 26 (3d10 + 10) bludgeoning damage.
  • Luring Scent. The jasconius unleashes an alluring, sweet scent. Every beast, humanoid, and giant within 300 feet of the jasconius that can smell the scent must succeed on a DC 15 Wisdom saving throw or be charmed until the scent ends. The jasconius must take a bonus action on its subsequent turns to continue releasing its scent. It can stop releasing its scent at any time. The scent ends if the jasconius is incapacitated. While charmed by the jasconius, a target is incapacitated and ignores the scent of other jasconius. If the charmed target is more than 5 feet away from the jasconius, the target must move on its turn toward the jasconius by the most direct route. It doesn’t avoid opportunity attacks, but before moving into damaging terrain, such as lava or a pit, and whenever it takes damage from a source other than the jasconius, a target can repeat the saving throw. A creature can also repeat the saving throw at the end of each of its turns. If a creature’s saving throw is successful, the effect ends on it. A target that successfully saves is immune to this jasconius’s scent for the next 24 hours.
  • Swallow. The jasconius makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the jasconius, and it takes 56 (16d6) acid damage at the start of each of the jasconius’s turns. If the jasconius takes 60 damage or more on a single turn from a creature inside it, the jasconius must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the jasconius. If the jasconius dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.


  • Whirlpool. As a bonus action after moving at least 10 feet, the jasconius can cause a whirlpool to form centered on itself. The whirlpool forms a vortex that is 5 feet wide at the base, up to 120 feet wide at the top, 50 feet tall, and lasts for 1 minute or until the wave shaper is incapacitated. Any creature or object in the water and within 50 feet of the vortex is pulled 10 feet toward it. A creature can swim away from the vortex by succeeding on a DC 26 Strength (Athletics) check. When a creature enters the vortex for the first time on a turn or starts its turn there, it must make a DC 26 Strength saving throw. On a failed save, the creature takes 17 (5d6) bludgeoning damage and is caught in the vortex until it ends. On a success, the creature takes half damage and isn’t caught in the vortex. A creature caught in the vortex can use its action to try to swim away from the vortex as described above, but it has disadvantage on the Strength (Athletics) check to do so. The first time each turn that an object enters the vortex, the object takes 36 (10d6) bludgeoning damage. This damage occurs each round it remains in the vortex.


The jasconius is a massive, aquatic monster that lurks in deep oceans, seas, and large freshwater bodies. By sitting still for long periods of time, it looks like an island. Jasconius can grow up to several miles in length, making them formidable predators of the deep.

Jasconius is primarily found in the deepest and most secluded parts of the ocean, where they patrol their vast territories. They occasionally venture close to shorelines, especially during storms, but most encounters with these elusive creatures happen in the midst of open waters.

The jasconius stays dormant for days at a time, allowing sea birds and crustaceans to land on it. It attracts other wildlife through its sweet scent, all giving the illusion that it is a habitable island. When enough creatures land on the jasconius, it dives and then turns around to chomp on everything created from its whirlpool. The jasconius is particularly sensitive to fire and anyone starting a campfire on its back will trigger its whirlpool attack.

When not hibernating, jasconius are apex predators, known for their cunning hunting strategies and power over water. They often lurk beneath the waves, using their aquatic camouflage to remain hidden from unsuspecting prey. Their immense strength allows them to capsize boats and drag entire crews into the depths.

Jasconius has a territorial nature and fiercely guards its domain against intruders. Other sea creatures, even large ones, often give the jasconius a wide berth, knowing better than to challenge this fearsome predator.

Legends tell of ancient mariners and sorcerers who were able to form uneasy alliances with jasconius, either through bribery, powerful spells, or mutual respect. Such relationships grant certain individuals protection against other sea monsters and the power to control the weather, but they come with great risks, as the jasconius is a creature of unpredictable nature and tempestuous moods.

Section 15: Copyright Notice

5E RPG: Celtic Bestiary. Copyright 2023, Mal and Tal Enterprises, LLC; Author Michael Tresca.

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