Amphibious.

The sea serpent can breathe air and water.

Legendary Resistance (2/Day).

If the sea serpent fails a saving throw, it can choose to succeed instead.

Siege Monster.

The sea serpent deals double damage to objects and structures.

Actions

Multiattack.

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

Bite.

Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 20 (2d12 + 7) piercing damage plus 6 (1d12) cold damage.

Constrict.

Melee Weapon Attack: +12 to hit, reach 20 ft., one creature. Hit: 29 (4d10 + 7) bludgeoning damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the sea serpent can't constrict another target.

Tail.

Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 13 (1d12 + 7) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be pushed up to 30 feet away from the sea serpent and knocked prone.

Rime Breath (Recharge 5–6).

The sea serpent exhales a 60-foot cone of cold. Each creature in that area must make a DC 18 Constitution saving throw, taking 49 (9d10) cold damage on a failed save, or half as much damage on a successful one.

Legendary Actions

The ancient sea serpent can take 3 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 ancient sea serpent regains spent legendary actions at the start of its turn.

Tail. The sea serpent makes one Tail attack.
Bite (Costs 2 Actions). The sea serpent makes one Bite attack

Ancient Sea Serpent