Keen Smell. The black troll has advantage on Wisdom (Perception) checks that rely on smell.
Amphibious. The black troll can breathe air and water.
Regeneration. The black troll regains 10 hit points at the start of its turn. If the troll takes 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.
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.
Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.
Acid Breath (Recharge 5–6). The black troll exhales acid in a 15-foot line that is 5 feet wide. Each creature in that line must make a DC 11 Dexterity saving throw, taking 22 (5d8) acid damage on a failed save, or half as much damage on a successful one.
Description
Particular trolls which have developed dark, almost black scaly skin. It resembles black dragon skin, as those particular trolls have been living in swamps and got occasional black dragon wyrmlings to eat. Such mutation developed immunity to one of two only damage types, trolls are weak to - acid. Besides immunity, skin gave more hard resistance to attacks and now these trolls have incredible acid in stomach. Exhaling stomach acid, they can make acid breath, which is quite fearsome move.
Black trolls mostly can be met in usual black dragon living place, marshes or swamps. They can breath underwater, so they keep hiding there, waiting for prey to come close.