Incorporeal Movement. The Wraith Sea Serpent can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Sunlight Sensitivity. While in sunlight, the Wraith Sea Serpent has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Siege Monster. The Wraith Sea Serpent deals double damage to objects and structures.

Multiattack. The Wraith Sea Serpent makes one bite attack and one tail attack.

Bite. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 20 (4d6 + 8) piercing damage.

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 13 (1d10 + 8) bludgeoning damage.

Create Specter. The Wraith Sea Serpent targets a humanoid within 10 feet of it that has been dead for no longer than 1 minute and died violently. The target's spirit rises as a specter in the space of its corpse or in the nearest unoccupied space. The specter is under the wraith's control. The wraith can have no more than seven specters under its control at one time.