Amorphous Everrot Sentinel

Swamp Walk. Difficult terrain composed of mud, shallow water, and water plants doesn't cost the Everrot Sentinel extra movement.

Swampy Camouflage. While the Everrot Sentinel remains motionless and prone in swampy terrain, it takes a DC 15 Wisdom (Perception) check to determine its presence.

Undead Nature. The Everrot Sentinel doesn't require air, food, drink, or sleep.

Multiattack. The Everrot Sentinel either makes two claw attacks or makes one Strangling Root attack and uses Expunge Madness if possible.

Claw. Melee Natural Attack: +8 to hit, range 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage, and the target must make a DC 14 Constitution saving throw or become poisoned for the next minute and take an extra 3(1d6) poison damage. On a success, the creature has advantage on saving throws against this feature for the next 24 hours.

Strangling Root. Ranged Natural Attack: +8 to hit, range 30 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage plus 10 (3d6) acid damage. If the target is Medium or smaller, it is grappled (escape DC 16). Until this grapple ends the target is paralyzed and the only action the target creature can take is to escape the grapple. The Everrot Sentinel has four such roots to grapple with.

Expunge Madness (Recharge 6). The Everrot Sentinel expels forth the spores, fumes, and rot that make up its innards in a 20-foot cone. Each creature in the area must make a DC 14 Constitution saving throw. On a failure, a creature takes 19 (3d12) poison damage and is poisoned for 1 minute. On a success, a creature takes half the damage and isn't poisoned. A poisoned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

It's Froggy Outside! When the Amorphous Everrot Sentinel is located within swampy or foggy terrain, it can use its bonus action to magically step through the terrain and reappear in another location within terrain that also fits the aforementioned requirements, essentially teleporting up to 30 feet away.

One with the Moss. If the Amorphous Everrot Sentinel is knocked prone while located in swampland terrain, then it can use its reaction to merge its form with the ground beneath it, sinking into the terrain itself. On its next turn, it can expend half of its movement to reappear using its It's Froggy Outside! feature. While it is submerged, it cannot be targeted by attacks or spells and it is considered incapacitated.