Keen Smell. The troll has advantage on Wisdom (Perception) checks that rely on smell.

Fear of Fire and Acid. While the troll is not actually susceptible to fire or acid, it is still deathly afraid of it. If they take fire or acid damage, the troll must make a DC 10 Wisdom saving throw. On a success, the troll gets as close as possible to the source of the damage (if its a creature) and do their very best to kill it, otherwise they run away. On a failed saving throw, they are frightened of the source until the end of their next turn.

Undead Fortitude. If damage reduces the Zombie Troll to 0 hit points, it must make a Constitution saving throw with a DC of 5+ the damage taken, unless the damage is radiant or from a critical hit.  On a success, the zombie troll drops to 1 hit point instead.

Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes radiant damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.

Power Crystal. When the troll takes necrotic damage, roll a d8. If you roll an 8, the troll takes no damage, as well as regaining necrotic pulse. The crystal has an AC of 21 and 27 (6d6) health with a vulnerability to bludgeoning damage and resistance to slashing damage. When the crystal is broken, Necrotic Pulse is immediately activated, and the troll undergoes the following changes:

• The troll loses its Regeneration feature, this feature, and the Necrotic Pulse attack.
• The troll gains the feature Rancid Degeneration
• It gains immunity to necrotic damage.

Rancid Degeneration (Only With a Broken Power Crystal). 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 radiant damage since the end of its last turn.

Multiattack. The troll makes three attacks: one with its bite and two with its claws, or uses the Necrotic Pulse attack, if it can.

Bite. Melee Weapon Attack: +8 to hit, reach 5ft., one target. Hit: 11 (1d8 + 4) piercing damage plus 14 (3d8) necrotic damage.

Claw. Melee Weapon Attack: +8 to hit, reach 5ft., one target. Hit: 11 (2d6 + 4) slashing damage plus 5 (1d8) necrotic damage.

Necrotic Pulse (Recharge 6). Each creature within 10 feet of the troll must make a DC 15 Dexterity saving throw, taking 27 (6d8) necrotic damage on a failed save, or half as much damage on a successful one. The troll heals itself an amount equal to half of the damage done by the Necrotic pulse. Creatures that were not aware of the Power Crystal are now aware of it, and know that the crystal is the source of the pulse.

This creature has 2 legendary actions that can be used at the end of another creatures turn. Expended legendary actions are regained at the beginning of the trolls turn.

Melee Attack. The Zombie troll uses Bite, or makes 2 Claw attacks.

Gather Energy (2 actions). The troll recharges its use of Necrotic Pulse