Amphibious.The Abyssal Merrow can breathe air and water

Doom Cluster. The Abyssal Merrow has advantage on an attack roll against a creature if at least one other ally is within 5 feet of the creature and the ally isn't incapacitated.

Swimming Leap. With a 10-foot swimming start, the Abyssal Merrow can long jump out of or across the water up to 25 feet.

Multiattack. Abyssal Merrow makes two attacks, one with Deep Trident and one with Water Spear.

Deep Trident. Melee or Ranged Weapon Attack: +9 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 9 (1d8,+ 5) piercing damage plus 13 (3d8) cold damage

Water Spear. Melee or Ranged Weapon Attack: +9 to hit, reach S ft. or range 20/60 ft., one target. Hit: 8 (1d6 + 5) piercing damage plus 13 (3d8) cold damage. Abyssal Merrow can only make this attack if its in contact with water.