Amphibious. The abyssal can breathe air and water.

Crustacean Speech. The abyssal can communicate simple concepts to crabs, lobsters, and other crustaceans or crustacean-like creatures when it speaks Demolith.

Demolith Subtype. The abyssal has the "(demolith)" tag, which has no rules of its own, but is a tag that affects the abyssal in the same way that it would affect a demon or devil.

Semi-Immortal Nature. The abyssal can't die of old age unless affected by magic of a spell of 6th level or higher and doesn't require food, drink, air, or sleep.

Multiattack. The abyssal makes three attacks; two with its trident and one with its tail. It can replace one of them with a Forceful Strike attack.

+1 Trident. Melee or Ranged Weapon Attack: +7 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 7 (1d6 + 4) piercing damage, or 8 (1d8 + 4) piercing damage if used with two hands to make a melee attack, plus 5 (1d10) acid damage.

Tail. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage.

Forceful Strike. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 60 ft., one target. Hit: 10 (2d6 + 3) acid, force, or cold damage (abyssal's choice).