Huge fiend (phlegethite), chaotic evil

Armor Class 18 (natural armor)
Hit Points 168 (16d12 + 64)
Speed 40 ft., swim 60 ft.

23 (+6) 14 (+2) 18 (+4) 10 (+0) 12 (+1) 10 (+0)

Skills Perception +7, Stealth +8
Damage Vulnerabilities psychic, radiant
Damage Immunities fire
Senses darkvision 120 ft., passive Perception 17
Languages Infernal
Proficiency Bonus +6
Challenge 19 (22,000 XP)


  • Amphibious. The cathach can breathe both air and water.
  • Bristled Hide. At the start of each of its turns, the cathach deals 7 (2d6) piercing damage to any creature grappling it.
  • Legendary Resistance (3/Day). If the cathach fails a saving throw, it can choose to succeed instead.
  • Siege Monster. The cathach deals double damage to objects and structures.
  • Susceptible to Holy Symbols. While the cathach is wearing or touching a holy symbol, it automatically fails saving throws against effects that turn Fiends.


  • Multiattack. The cathach makes three attacks: one with its Bite and two with its Claws.
  • Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 7 (2d6) fire damage, and the target is swallowed if it is a Medium or smaller creature. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the cathach, and takes 10 (3d6) acid damage at the start of each of the cathach’s turns. The cathach’s gullet can hold up to two creatures at a time. If the cathach takes 20 damage or more on a single turn from a creature inside it, the cathach must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls prone in a space within 10 feet of the cathach. If the cathach dies, any swallowed creature is no longer restrained by it and can escape from the corpse using 10 feet of movement, exiting prone.
  • Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.
  • Tail Slap. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) piercing damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.
  • Fire Breath (Recharge 5-6). The cathach exhales fire in a 60-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 42 (12d6) fire damage on a failed save, or half as much damage on a successful one.


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

  • Tail. The cathach makes one Tail Slap attack.
  • Bite (Costs 2 Actions). The cathach makes one Bite attack.
  • Boiling Aura (Costs 3 Actions). The cathach radiates intense heat. Until the start of the cathach’s next turn, whenever a creature starts its turn within 20 feet of the cathach, that creature must succeed on a DC 18 Constitution saving throw or take 40 (9d8) fire damage. Being underwater doesn’t grant resistance against this damage.


The cathach is a massive and fearsome monster. Its back is covered with scales and shells, resembling a round island. A rough, bristly mane runs along its foreparts, and its cavernous mouth is filled with a double row of sharp teeth, revealing its entrails when opened. Its single eye gleams with cruelty, and its body is supported by two short, thick, hairy legs armed with iron nails. The creature’s tail, like that of a whale, ends in iron spikes that can plow furrows in the ground as it moves.

The cathach (kah-TAK) is equally at home on land and in the water, capable of moving and swimming with ease. Its presence causes the sea to boil around it.

Cathach are known for their destructive rampages. They lay waste to the surrounding land, sinking ships and disrupting commerce. They do have a weakness, vulnerable to holy commands and attacks.

One of the few creatures native to the Seven Circle of Infernus’ burning river, the Pyriphlegethos, the cathach is a feared predator even there.

Section 15: Copyright Notice

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

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