Huge monstrosity, chaotic evil

Armor Class 18 (natural armor)
Hit points 243 (18d12 + 126)
Speed 0 ft., swim 40 ft.

25 (+7) 10 (+0) 25 (+7) 12 (+1) 16 (+3) 16 (+3)

Skills Perception +18, Stealth +7
Saving Throws Str +14, Con +14
Damage Immunities poison
Condition Immunities frightened, poisoned
Senses blindsight 60 ft., darkvision 120 ft., passive Perception 28
Languages Aquan, Draconic
Proficiency Bonus +7
Challenge 22 (41,000 XP)


  • Barbed Hide. At the start of each of its turns, the muirdris deals 5 (1d10) piercing damage to any creature grappling it.
  • Legendary Resistance (3/Day). If the muirdris fails a saving throw, it can choose to succeed instead.
  • Water Breathing. The muirdris can breathe only underwater.


  • Multiattack. The muirdris makes three attacks: one with its Bite and two with its Poison Quills.
  • Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage or 29 (4d10 +7) when enlarged plus 7 (2d6) poison damage and the target is grappled (escape DC 22). Until this grapple ends, the target is restrained, and the muirdris can’t bite another target.
  • Poison Quills. Melee Weapon Attack: +14 to hit, reach 5 ft., one creature. Hit: 12 (1d10 + 7) poison damage or 18 (2d10 + 7) poison damage when enlarged, and the target must succeed on a DC 22 Constitution saving throw or be poisoned for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
  • Vomit (Recharge 5-6). The muirdris exhales its stomach contents in a 60-foot cone. Each creature in that area must make a DC 15 Dexterity saving throw, taking 54 (12d8) poison damage on a failed save, or half as much damage on a successful one.
  • Swallow. The muirdris makes one bite attack against a Large or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the muirdris, and it takes 21 (6d6) acid damage at the start of each of the muirdris’s turns. A muirdris can have only one creature swallowed at a time. If the muirdris takes 30 damage or more on a single turn from the swallowed creature, the muirdris must succeed on a DC 22 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the muirdris. If the muirdris dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.
  • Frightful Presence. Each creature of the muirdris’s choice that is within 120 feet of the muirdris 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 muirdris’s Frightful Presence for the next 24 hours.
  • Enlarge (Recharges after a Short or Long Rest). For 1 minute, the muirdris magically increases in size, along with anything it is wearing or carrying. While enlarged, the muirdris is Gargantuan, doubles its damage dice on Strength-based weapon attacks (included in the attacks), and makes Strength checks and Strength saving throws with advantage. If the muirdris lacks the room to become Gargantuan, it attains the maximum size possible in the space available.


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

  • Poison Spines. The muirdris makes a Poison Quill attack.
  • Move. The muirdris moves up to half its speed.
  • Duiderc (Costs 2 Actions). The muirdris targets up to three creatures it can see within 120 feet of it. Each target must make a DC 22 Constitution saving throw. On a successful save, a creature becomes immune to this muirdris’ Duiderc. On a failed save, the target is poisoned, gains 1 level of exhaustion, and its Charisma score is reduced by 1d4. The target dies if this reduces its Charisma to 0. While poisoned in this way, the target must repeat the saving throw at the start of each of its turns. Three successful saves against the poison end it, and ending the poison removes any levels of exhaustion caused by it. Each failed save causes the target to gain another level of exhaustion. Once the target reaches 6 levels of exhaustion, it dies. The transformation of the body and Charisma drain can be undone only by a wish spell.


The muirdris appears as a large fish with a body covered in shimmering scales that display a mesmerizing array of colors. Its sleek body is adorned with bold, contrasting stripes in shades of electric blue, fiery orange, and deep purple. The muirdris possesses an array of long, venomous spines that protrude from its back and sides, resembling the dangerous appendages of a lionfish.

The muirdris is a legendary spined sea creature that possesses a unique ability to manipulate its body, swelling up to great size and deflating again at will. The muirdris is known for its thorny branches and stings that strike fear into the hearts of those who have encountered it.

The muirdris is a relentless predator, emerging from the depths to hunt its prey. It possesses a combination of physical prowess and its frightful presence, capable of instilling paralyzing fear in its victims. Those who fail to resist its terrifying aura may find themselves warped and twisted.

The muirdris is a formidable adversary, preying upon unsuspecting creatures in the surrounding area. It uses its powerful jaws and claws to tear through flesh and scales, leaving devastation in its wake.

Section 15: Copyright Notice

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

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