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

Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire 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.

Infestation. The troll is riddled with millions of tiny rot grubs. When the troll hits a creature with an attack on its turn, the target must make a DC 10 Dexterity saving throw; on a failed save, 1d4 rot grubs drop onto it and latch on, causing 1 point of piercing damage per grub.

A rot grub is a Tiny beast with AC 6, 1 hit point, a 2 (-4) in every ability score, and a speed of 1 foot. While on the target’s skin, a grub can be killed by normal means or quickly burned off using any flame (which causes 1 hp of fire damage to the victim, but destroys all grubs currently latched onto it).

At the start of the troll's next turn, any grubs still latched onto a victim burrow under the target’s skin dealing 4 (1d6) piercing damage per grub (maximum 4d6). Burrowing grubs have total cover, and proceed to burrow their way towards the victim's heart.

From this point on, the grubs can only be removed by a spell or other effect that cures poison or disease. Such an effect instantly destroys all grubs currently infesting the target.

At the end of each of its turns after the grub burrows in, the target takes 4 (1d6) piercing damage per grub currently infesting it (maximum 4d6) until the grubs are removed or the target drops to 0 hit points. If it drops to 0 hit points before all grubs are removed, the target dies.

When the troll starts its turn at 0 hit points and doesn't regenerate, it dies and all remaining rot grubs emerge from the carcass, forming a medium Swarm of Rot Grubs.

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

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage, plus the target is subject to the troll's Infestation trait, as described under Special Traits above.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage, plus the target is subject to the troll's Infestation trait, as described under Special Traits above.