Armor Class 18 (natural armor)
Hit Points 472 (27d20 + 189)
Speed 20 ft., swim 60 ft.
Saving Throws STR +18, DEX +8, CON +15, INT +14, WIS +12
Damage Immunities lightning, [object Object]
Condition Immunities frightened, paralyzed
Senses truesight 120 ft., passive Perception 14
Languages understands Abyssal, Celestial, Infernal, and Primordial but can't speak, telepathy 120 ft.
Challenge 25
Legendary Resistance (3/Day). If Slarkrethel fails a saving throw, it can choose to succeed instead.
Amphibious. Slarkrethel can breathe air and water.
Freedom of Movement. Slarkrethel ignores 3, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.
Siege Monster. Slarkrethel deals double damage to objects and structures.
Multiattack. Slarkrethel makes three tentacle attacks, each of which it can replace with one use of Fling.
Bite. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 10) piercing damage. If the target is a Large or smaller creature grappled by Slarkrethel, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has 3 against attacks and other effects outside Slarkrethel, and it takes 42 (12d6) acid damage at the start of each of Slarkrethel's turns. If Slarkrethel takes 50 damage or more on a single turn from a creature inside it, Slarkrethel must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Slarkrethel. If Slarkrethel dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.
Tentacle. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. Slarkrethel has ten tentacles, each of which can grapple one target.
Fling. One Large or smaller object held or creature grappled by Slarkrethel is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone.
Lightning Storm. Slarkrethel magically creates three bolts of lightning, each of which can strike a target Slarkrethel can see within 120 feet of it. A target must make a DC 23 Dexterity saving throw, taking 22 (4d10) lightning damage on a failed save, or half as much damage on a successful one.
Tentacle Attack or Fling. Slarkrethel makes one tentacle attack or uses its Fling.
Lightning Storm (Costs 2 Actions). Slarkrethel uses Lightning Storm.
Ink Cloud (Costs 3 Actions). While underwater, Slarkrethel expels an ink cloud in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than Slarkrethel. Each creature other than Slarkrethel that ends its turn there must succeed on a DC 23 Constitution saving throw, taking 16 (3d10) poison damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of Slarkrethel's next turn.
SKT, p. 224