Aquatic Drowning Troll

Amphibious. The troll can breath air and water.

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

Pack Tactics. The troll has advantage on an attack roll against a creature if at least one of the troll's allies is within 5 feet of the creature and the ally doesn't have the incapacitated condition. 

Aquatic Grappler. The troll doesn't have its Speed reduced while moving through water with a creature Grappled by it.

Regeneration. The troll regains 10 Hit Points at the start of each of its turns. If the troll takes Acid or Fire damage, this trait doesn’t function on 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

Claw. Melee Attack Roll: +7, reach 10 ft. Hit: 11 (1d6 + 4) Slashing damage.

Bite. Melee Attack Roll: +8, reach 5 ft. Hit: 9 (1d8 + 4) Piercing damage.

Sieze. Melee Attack Roll: +7, reach 10 ft. Hit: 11 (1d6 + 4) Slashing damage and a large or smaller target has the Grappled condition (escape DC15) until the end of their next turn.

Drag Under. The troll moves up to half of its swim speed without provoking an opportunity attack from the creature it Grappled. The Grappled creature moves with it. If this movement takes the target completely underwater, the troll may immediately move an additional 10 feet downward, provide the water is deep enough. 

Death Grip. When a creature Grappled by the troll successfully escapes, the troll can make one Claw attack against that creature.