Deep Sea Ormr

Deep Sea Ormr CR: 15 (13,000 XP)

Gargantuan ormr, unaligned

Armor Class: 18 (natural armor)

Hit Points: 350 (20D20 +140)

Speed: , swim: 50 ft

STR

25 +7

DEX

10 +0

CON

23 +6

INT

5 -3

WIS

15 +2

CHA

6 -2

Saving Throws: Strength +12, Constitution +10

Skills: Athletics +11, Perception +5, Stealth +3

Damage Resistances: bludgeoning, piercing, and slashing from nonmagical attacks

Condition Immunities: Frightened, Restrained

Senses: darkvision 120ft., passive Perception 15

Challenge Rating: 15 (13,000 XP)

Hold Breath. The Ormr can hold its breath for 24 hours.
Siege Monster. The ormr deals double damage to buildings and structures.

Actions

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 23 (3d10+7) piercing damage. If the target is a creature, it is grappled (escape DC 17). Until this grapple ends, the target is restrained, and the sea serpent can't bite another target.
Tail. +11 to hit:, 20ft., , one target. Hit 39(8d8+7) bludgeoning damage. If the target is a creature, it must succeed a DC 19 Constitution saving throw or be stunned until the end of its next turn.
Swallow. The sea serpent makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, that creature takes the bite's damage and is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the sea serpent, and it takes 21 (6d6) acid damage at the start of each of the ormr's turns.
If the ormr takes 30 damage or more on a single turn from a creature inside it, the ormr must succeed on a DC 15 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 sea serpent's head. If the sea serpent dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 20 feet of movement, exiting prone.
Ensnare Vessel. The ormr targets a gargantuan or smaller ship. The ship's helmsman must make a water vehicle check contested by the ormr's strength check. If the ormr succeeds the ship is grappled and may not move. Each round, at the beginning of the ormr's turn, a ship grappled in this way takes 36 (2d10+7, doubled for siege monster) bludgeoning damage as the ormr crushes it.
Once per round a person at the ship's helm may make a DC 17 water vehicles check. If the ship wins this check it is no longer grappled and may move normally.

Legendary Actions

The ormr can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The ormr regains spent legendary actions at the start of its turn.
Bite. The sea serpent makes one bite attack.
Bash. The sea serpent targets an adjacent 20 ft cube. Each creature in the area must make a DC 17 strength saving throw or take 12 (1d8+7) damage and either be knocked prone or pushed 10 feet in a direction of the sea serpent's choice as the serpent slams into them. A creature that succeeds on its saving throw takes half damage and suffers no other effect. All creatures experience the same secondary effect, either knocked prone or shoved in the same direction.
Swallow (Costs 2 Actions): The sea serpent uses its Swallow.