Olhydra

Huge elemental, neutral evil
Armor Class 18 (natural armor)
Hit Points 324 (24d12 + 168)
Speed 50 ft., swim 100 ft.
STR
21 (+5)
DEX
22 (+6)
CON
24 (+7)
INT
17 (+3)
WIS
18 (+4)
CHA
23 (+6)
Saving Throws Str +11, Con +13, Wis +10
Damage Resistances lightning; bludgeoning, piercing, and slashing damage from nonmagical attacks
Damage Immunities acid, cold, poison
Condition Immunities charmed, frightened, paralyzed, petrified, poisoned, prone, restrained
Senses blindsight 120 ft., passive Perception 14
Languages Aquan
Challenge 18 (20000 XP)
Empowered Attacks.
Innate Spellcasting.
Legendary Resistance (3/Day).
Magic Resistance.
Water Form.
ACTIONS
Multiattack.
Slam.
Water Jet.
Summon Elementals (1/Day).
LEGENDARY ACTIONS
Crush.
Fling (Costs 2 Actions).
Water to Acid (Costs 3 Actions).

This full creature's stat block is not available (not OGL).

Olhydra is the Princess of Evil Water. Sometimes known as the Crushing Wave, the Dark Tide, or the Well of Endless Anguish, she takes the form of a great wave.
Adventures (Princes of the Apocalypse)