Hold Breath. The Ancient Hydro-Serpent can hold its breath for 12 hours.
Underwater Camouflage. The Ancient Hydro Serpent has advantage on Dexterity (Stealth) checks made while underwater.
Brute. A natural weapon attack deals one extra die of its damage when the Ancient Hydro-Serpent hits with it.
Siege Monster. The Ancient Hydro-Serpent deals double damage to objects and structures.
Limited Magic Resistance.The Ancient Hydro-Serpent can't be affected or detected by spells of 3rd level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.
Legendary Resistance (3/Day). If the serpent fails a saving throw, it can choose to succeed instead.
Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 8) Piercing damage. The target is then Grappled with an escape DC of 20. Until the grapple ends, the creature is restrained and the young sea serpent can't bite another target.
Constrict. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 28 (4d8 + 8) Bludgeoning damage, and the target is Grappled with an escape DC 20. Until this grapple ends, the creature is restrained, and the serpent can still constrict another target.
Swallow. The Ancient Hydro-Serpent makes one bite attack against a large or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is Blinded and Restrained, it has total cover against attacks and other effects outside the frog, and it takes 15 (3d8) Acid damage at the start of each of the frog's turns. If the Ancient Hydro-Serpent takes more than 40 points of damage from a creature inside it, the serpent must succeed on a DC 22 Constitution saving through at the end of that turn regurgitate all swallowed creatures, which fall Prone in a space within 10 feet of the serpent. The Ancient Hydro-Serpent can have up to 5 creatures swallowed at a time and if the Ancient Hydro-Serpent dies, a swallowed creature(s) is no longer Restrained by it and can escape from the corpse using 20 feet of movement, exiting Prone.
Frigid Breath (Recharge 5–6). The Sea Serpent exhales freezing gas in a 30-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 84 (12d12) Cold damage on a failed save, or half as much damage on a successful one.
The Ancient Hydro-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 serpent regains spent legendary actions at the start of its turn.
Body Slam (Costs 2 Actions). The serpent rears its massive body back before slamming all of its weight down. Each creature within 30 feet of the serpent must succeed on a DC 25 Dexterity saving throw or take 27 (3d8 +12) Bludgeoning damage and be knocked Prone.
Bellowing Roar. The serpent roars, shaking the earth, air and sea around it within 150 feet. Each creature aware of the Ancient Hydro-Serpent must make a DC 23 Charisma saving throw or become Frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the serpents Bellowing Roar for the next 24 hours.
Description
Ancient hydro-serpents can be longer than ships, and they hunt massive prey.
They rarely surface except to breathe and for one specific month a year, in which they gorge themselves on as much food as they can, which includes the entire crews of ships.
Lair and Lair Actions
An Ancient Hydro-Serpent's Lair
Ancient Hydro-Serpents make their lair deep in the depths of the ocean, where it is cold and hard to breath and both food and light is almost non-existent. Its lair is easily identified by the hundreds of ship husks around the large, winding tunnels and geo-thermal vents. More husks can be found scattered within these tunnels, with several more reduced to scrap in order to make up the Ancient Hydro-Serpent's nest.
Lair Actions
On initiative count 20 (losing initiative ties), the Ancient Hydro-Serpent takes a lair action to cause one of the following effects; the Ancient Hydro-Serpent can’t use the same effect two rounds in a row:
- Extremely hot water erupts from the ground the serpent can see within 80 feet of it, creating a 40-foot high, 10-foot radius geyser. Each creature in the geyser's area must make a DC 18 Dexterity saving throw, taking 18 (3d10) Fire damage on a failed save, or half as much on a success.
- A tremor shakes the water-filled lair in a 60-foot radius around the serpent. Each creature other than the serpent in the water in that area must succeed on a DC 18 Constitution saving throw or become Stunned.
- A freezing, ice-laced liquid fills a 80-foot radius cube centering around the Ancient Hydro-Serpent and lightly obscures the area. It lasts until initiative count 20 on the next round. Each creature that starts its turn in the cloud will take 18 (3d10) Cold damage and continues to take this damage for the duration.
Regional Effects
The oceans containing an Ancient Hydro-Serpent's lair is warped by the serpent's mere presence, which creates one or more of the following effects:
- Small earthquakes and large whirlpools are common within 5 miles of the serpent's lair.
- Water sources within 1 mile of the lair is unnaturally cold, even if the rest of the water is nice and temperate.
- Jutting, underground spires within 3 miles of the lair form portals to the Elemental Plane of Water and the Para-Elemental Plane of Ice, allowing creatures of elemental water and ice into the world to dwell nearby.
If the Ancient Hydro-Serpent dies, these effects fade over the course of 1d10 days.
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