Gargantuan monstrosity, neutral

Armor Class: 17 (natural armor)

Hit Points: 181 (11d20 + 66) 11d20+66

Speed: 20 ft , swim: 60 ft

STR

23 +6

DEX

10 +0

CON

22 +6

INT

12 +1

WIS

13 +1

CHA

10 +0

Saving Throws: Dex +4, Con +11, Wis +5

Skills: Arcana +5, History +5, Perception +5

Damage Immunities: cold

Senses: darkvision 60 ft., passive Perception 15

Languages: Common, Draconic

Challenge Rating: 12

Pearl Lake Ancient by Richard Wright

Actions

Multiattack. The serpent makes two attacks: one with its bite and one with its tail.
Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 22 (3d10 + 6) piercing damage.
Tail. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 17 (2d0 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 16 Strength saving throw or be knocked prone.
Swallow. The serpent makes one bite attack against a Medium 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 serpent, and it takes 21 (6d6) acid damage at the start of each of the serpent's turns. A serpent can have only one creature swallowed at a time. If the serpent takes 30 damage or more on a single turn from the swallowed creature, the serpent must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the serpent. If the serpent dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.