Death Burst. When the swamp thing is reduced to 0 hp by lightning damage, it explodes in a burst of viscera. Each creature within 30 feet of it must make a DC 10 Dexterity saving throw, taking 8 (2d8) acid damage on a failed save, or half as much damage on a successful one.

Limited Amphibiousness. The swamp thing can breathe air and water, but needs to be submerged in the sea at least once a day for 1 minute to avoid suffocating.

Ranine Anatomy. The swamp thing can move almost silently in swampy environments. It has expertise in the Stealth skill when moving through water or muddy terrain. 

Multiattack. The swamp thing makes two attacks with its claws and one with its tongue.

Claws. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) slashing damage.

Tongue Strike. Melee Weapon Attack: +4 to hit, reach 10 ft., one target. Hit: 4 (1d4 + 2) acid damage.

Tongue Garrote. Melee Weapon Attack: +10 to hit, reach 5 ft., one Medium or Small creature against which the swamp thing has advantage on the attack roll. Hit: 4 (1d4 + 2) acid damage, and the target is grappled (escape DC 13). Until this grapple ends, the target is restrained and choking, and can't breathe or speak. The escape DC increases by 2 for each additional swamp thing grappling the target.

Throttle. When a swamp thing is damaged by an attack while grappling a creature, it can use its reaction to tighten its grip on the creature it is choking. The choking creature must make a DC 13 Constitution saving throw. On a failed save, the number of rounds before it runs out of breath decreases by 1.