Undead Nature. The sea serpent does not need to breath, eat or sleep.

Siege Monster. The sea sepent deals double damage to objects and structures.

Multiattack. The sea serpent makes one Bite attack and one Constrict or Tail attack.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 5 (1d10) cold damage.

Constrict. Melee Weapon Attack: +7 to hit, reach 20 ft., one target. Hit: 22 (4d8 + 4) bludgeoning damage. If the target ist Large or smaller, it ist grappled (escape DC 15). Until this grapple ends, the target ist restrained, and the sea serpent can't cosntrict another target.

Tail. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 9 (1d10 + 4) bludgeoning damage. If the target is a creature it must succeed on a DC 15 Strength saving throw or be pushed up to 20 feet away and knocked prone.

 Rime Breath (Recharge 5–6). The sea serpent exhales a 30-foot cone of cold. Each creature in that area must make a DC 14 Constitution saving throw, taking 38 (7d10) cold damage on a failed save, or half as much damage on a successful one.