Gargantuan monstrosity (shapechanger), unaligned

Armor Class 18 (natural armor)
Hit Points 396 (24d20 + 144)
Speed 40 ft., swim 60 ft.

30 (+10) 14 (+2) 22 (+6) 6 (-2) 10 (+0) 7 (-2)

Skills Perception +8, Stealth +10
Damage Resistances bludgeoning, piercing, and slashing from nonmagical attacks
Condition Immunities charmed, frightened, paralyzed, poisoned
Senses darkvision 120 ft., passive Perception 18
Proficiency Bonus +8
Challenge 26 (90,000 XP)


  • Amphibious. The ceirean can breathe air and water.
  • Keen Smell. The ceirean has advantage on Wisdom (Perception) checks that rely on smell.
  • Magic Resistance. The ceirean has advantage on saving throws against spells and other magical effects.
  • Legendary Resistance (3/Day). If the ceirean fails a saving throw, it can choose to succeed instead.


  • Multiattack. The ceirean makes two attacks: one with its Bite and one with its Tail.
  • Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 33 (4d10 + 10) piercing damage. If the target is a Huge or smaller creature, it must succeed on a DC 20 Dexterity saving throw or be swallowed. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the ceirean, and it takes 56 (16d6) acid damage at the start of each of the ceirean’s turns. If the ceirean takes 60 damage or more on a single turn from a creature inside it, the ceirean 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 ceirean. If the ceirean dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.
  • Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 28 (4d8 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.
  • Frightful Presence. Each creature of the ceirean’s choice within 60 feet of it and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the ceirean’s Frightful Presence for the next 24 hours.
  • Squash. Melee Weapon Attack: +18 to hit, reach 15 ft., one Huge or smaller creature. Hit: 37 (6d8 + 10) bludgeoning damage, the ceirean lands prone in the target’s space, and the target is grappled (escape DC 26). Until this grapple ends, the target is prone. The grapple ends early if the ceirean stands up.


  • Change Shape. The ceirean can polymorph into a Tiny fish or back into its true form. Its statistics are the same in both forms, except it retains its size.


The ceirean 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 ceirean regains spent legendary actions at the start of its turn.

  • Bite. The ceirean makes a bite attack.
  • Tail Swipe. The ceirean makes a tail attack.
  • Chomp (Costs 2 Actions). The ceirean makes one bite attack or uses its Swallow.


The ceirean resembles a colossal serpentine beast. Its body is covered in thick, slimy scales, ranging in deep purple to light blue, allowing it to blend in with its surroundings. The creature measures up to 60 feet in length and possesses a powerful tail that propels it through the water with remarkable speed and agility.

The ceirean (SEER-e-AN) is a fearsome sea creature that dwells in coastal waters. It is known for its predatory nature, often rising up from the depths to devour merfolk, whales, sharks, and any other unfortunate creatures that come within its reach.

The head of the ceirean bears sharp teeth with small, beady eyes that gleam ominously in the darkness. It emits a chilling screech that echoes through the water, instilling fear in all who hear it.

The most dangerous aspect of a ceirean is its size. They can transform into typical specimens of fish, and then morph back to their gargantuan true forms. The consequences for sailors are disastrous as the ceirean obliterates their boat. For predators who have swallowed a ceirean, the end result is likely death.

Encountering a ceirean is a terrifying prospect, as few creatures can withstand its massive size, powerful attacks, and ability to induce fear with its presence alone. Sailors and seafarers speak of the creature with trepidation, for crossing paths with a ceirean often leads to a swift and gruesome demise.

Section 15: Copyright Notice

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

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