Wriggling Beast

Rotting Skin. When the Wriggling Beast is at 50% of its hit points or below, and is hit by a melee attack the creature that attacked it takes 1d6 poison damage. 

Severed Arm. When the Wriggling Beast reaches 50% hit points its snake arm is severed subtracting 2 from its armor class. When the arm is severed the Wriggling Beast makes two Poison Spray attacks, no longer making Arm Swipe or Arm Bite attacks. 

Multiattack. The Wriggling Beast makes two Arm Swipe attacks or one Arm Bite attack. When the Wiggling Beast's arm is severed, it instead make two Poison Spray attacks.

Arm Swipe. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d8 + 1) bludgeoning damage.

Arm Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 3) piercing damage, and the target must make a DC 8 Constitution saving throw, taking 6 (3d4) poison damage, or half as much damage on a successful one. If the target failed the save they are poisoned for 1 minute, the target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Poison Spray. Ranged Weapon Attack: +3 to hit, range 10 ft., one creature. Hit: The target must make a DC 13 Constitution saving throw, taking 12 (2d12) poison damage on a failed save, or half as much damage on a successful one. If the target failed the save they are poisoned for 1 minute, the target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.