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.
Lightning Conduit. If the sea serpent is subjected to lightning damage, it is unhurt, and the lightning damage is instead divided evenly among all creatures within 5 feet of the sea serpent. In addition, the sea serpent recharges its Lightning Breath.
Siege Monster. The sea serpent 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: +12 to hit, reach 15 ft., one target. Hit: 20 (2d12 + 7) piercing damage plus 6 (1d12) lightning 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.
Lightning Breath (Recharge 5–6). The sea serpent exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that area must make a DC 18 Dexterity saving throw, taking 49 (9d10) lightning damage on a failed save, or half as much damage on a successful one.
The sea serpent can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The 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
Armor of Storms (Costs 3 Actions). Lightning temporarily surrounds the sea serpent, and it gains 40 temporary hit points until the start of its next turn. Until all these temporary hit points are gone, any creature that touches the sea serpent or hits it with a melee attack takes 26 (4d12) lightning damage.
Lair and Lair Actions
A Sea Serpent’s Lair
A sea serpent lives in dark depths, usually a sunken rift or a cavern filled with detritus, treasure, and wrecked ships.
Lair Actions
On initiative count 20 (losing initiative ties), the sea serpent takes a lair action to cause one of the following magical effects:
- A strong current moves through the sea serpent’s lair. Each creature within 60 feet of the sea serpent must succeed on a DC 23 Strength saving throw or be pushed up to 60 feet away from the sea serpent. On a success, the creature is pushed 10 feet away from the sea serpent.
- Creatures in the water within 60 feet of the sea serpent have vulnerability to lightning damage until initiative count 20 on the next round.
- The water in the sea serpent’s lair becomes electrically charged. All creatures within 120 feet of the sea serpent must succeed on a DC 23 Constitution saving throw, taking 10 (3d6) lightning damage on a failed save, or half as much damage on a successful one.
Regional Effects
The region containing a sea serpent’s lair is warped by the creature’s presence, creating the following magical effects:
- Thunderstorms rage within 6 miles of the lair.
- Water elementals coalesce within 6 miles of the lair. These elementals can’t leave the water and have Intelligence and Charisma scores of 1 (−5).
- Aquatic creatures within 6 miles of the lair that have an Intelligence score of 2 or lower are charmed by the sea serpent and aggressive toward intruders in the area.
When the sea serpent dies, all of these regional effects fade immediately.
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