Rancid Degeneration. At the end of each of the troll’s turns, each creature within 5 feet of it takes 11 (2d10) necrotic damage, unless the troll has taken acid or fire damage since the end of its last turn.

Flailing Limbs. If a melee attack to hit is over 21, a limb is cleaved from the troll, limiting its attack by one. 

Multiattack. The troll makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage plus 16 (3d10) necrotic damage.

Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage plus 5 (1d10) necrotic damage.

Bonus Action. Horrific Healing. If a limb has been severed, two limbs grow in its place. The troll immediately makes a claw attack with its new limb and its multi-attack grows by one. 

Villain Reactions

Round 1: 'So you's wants to play' - The Troll can leap 20 feet without provoking an opportunity attack. Enemies within 10 ft. of where it lands must make a DC 15 DEX saving throw or be knocked prone and take 2d10 damage. 

Round 2: 'No pointies' - The Troll makes a melee attack against an enemy within 5 ft of it, upon a hit disarms and throws the weapon 20/60 ft at another enemy. +5 to hit, and deals appropriate weapon damage +4. 

Round 3: 'Nos Nos Nos'  - The Troll howls in rage. Enemies must make a DC 14 Wisdom saving throw or be frightened for 1 round.