Homebrew — Original adaptation of the Nuckelavee from Orcadian folklore.

Traits

Amphibious. The Nuckelavee can breathe air and water.

Fresh Water Intolerance. If the Nuckelavee is splashed with fresh water (minimum 1 cup) it must succeed on a DC 15 Wisdom Saving Throw or be stunned until the end of its next turn. This effect may successfully stun it up to 5 times/day. After the fifth successful stun, it automatically succeeds on subsequent saves against this effect.

Legendary Resistance (3/Day). If the Nuckelavee fails a saving throw, it can choose to succeed instead.

Nocturnal creature. At night the Nuckelavee is alert and has Proficiency in Dexterity Saving Throws (+8) and Wisdom Saving Throws (+6).

Pestilential Aura. The Nuckelavee emanates an aura in a 20-foot Emanation. Any creature that enters or starts its turn in the aura must succeed on a DC 16 Constitution Saving Throw or take 3d6 Necrotic damage. Creatures with resistance to necrotic damage automatically succeed on this saving throw. Plants without resistance to Necrotic damage automatically fail this saving throw and gradually wilt, wither and die while within the aura. Opening any healing potion within this aura makes the potion immediately and permanently spoil. A creature can be affected by this aura only once per turn.

Actions

Multiattack. The Nuckelavee makes two Claw attacks and one Hooves attack or three Claw attacks or one Take to the Sea Attack.

Claw. Melee Attack Roll: +8, reach 5 ft. Hit: 14 (3d6 + 4) Slashing damage.

Hooves. Melee Attack Roll: +8, reach 5 ft. Hit: 11 (3d4 + 4) Bludgeoning damage. If the target is a Large or smaller creature and the Nuckelavee moved 20+ feet straight toward it immediately before the hit, the target takes an extra 7 ( 3d4 ) Bludgeoning damage and has the Prone condition.

Take to the Sea. Melee Attack Roll: +8, reach 5 ft. Hit: 14 (3d6 + 4) Piercing damage. If the target is a Medium or smaller creature it must make a DC 16 Strength or Dexterity Saving Throw (target's choice). On a failure, the target is scooped up and placed on the Nuckelavee and grappled. While it has a grappled target, the Nuckelavee will carry its target towards the nearest body of salt water and attempt to drown its target. While carrying a grappled creature, the Nuckelavee, as its action, can only make one Claw attack per turn. This attack has advantage against the creature it is currently carrying. At the start of its turn the grappled creature may make a DC 16 Strength Saving Throw to end the grapple and escape from the Nuckelavee. If this succeeds, the creature must also make a DC 16 Dexterity Saving Throw. On a failure the target falls prone next to the Nuckelavee instead of landing upright.

Bonus Actions

Screech. All targets within 60 ft. of the Nuckelavee make a DC 16 Wisdom Saving Throw, a Constitution Saving Throw, or both, as chosen by the GM. The Nuckelavee can then apply either or both of the following effects to creatures that fail the corresponding save:
Fear. A creature that fails the Wisdom saving throw has the Frightened condition until the start of the Nuckelavee's next turn.
Constant Drone. A creature that fails the Constitution saving throw has a −2 penalty to Dexterity-based rolls and AC until the start of the Nuckelavee's next turn. The creature loses concentration and cannot cast spells requiring concentration during this time.
GM Note. Screech is intended as a real-time difficulty adjustment. Use it less frequently if the party is struggling.

Reactions

Legendary Back Kick. Melee Attack Roll: +8, reach 5 ft. Hit: 11 (3d4 + 4) Bludgeoning damage. If a creature moves into a space within 5 ft. directly behind the Nuckelavee, the Nuckelavee may kick the target. If the attack is successful and the target is a Large or smaller creature then the target must succeed on a DC 16 Strength Saving Throw or be thrown 10 ft. and fall prone. The Nuckelavee can use this reaction multiple times per round, but only once per triggering creature's movement. This reaction allows the Nuckelavee to turn in place (without moving to a new space) so that the triggering creature is within its Back Kick triggering area.