Bloodfin

Water Breathing. The Bloodfin can breathe only underwater.

Keen Senses. The Bloodfin has advantage on Wisdom (Perception) checks that rely on smell or sight.

Aquatic Camouflage. The Bloodfin has advantage on Dexterity (Stealth) checks made to hide in dimly lit underwater environments.

Pack Tactics. The Bloodfin has advantage on attack rolls against a creature if at least one of the Bloodfin's allies is within 5 feet of the creature and the ally isn’t incapacitated.

Blood Frenzy. The Bloodfin has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Lunge. The Bloodfin can move up to its speed towards a creature it can see, making a Bite attack as part of this movement. If the attack hits, the target must make a DC 14 Dexterity saving throw or be grappled.

Multiattack. The Bloodfin makes two attacks: one with its Bite and one with its Tail Slash.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) piercing damage.

Tail Slash. Melee Weapon Attack: +4 to hit, reach 10 ft., one target. Hit: 8 (1d8 + 4) bludgeoning damage.

Swallow. If the Bloodfin has grappled a Medium or smaller target, the Bloodfin can swallow the target as an action. The target is blinded and restrained, and it takes 10 (3d6) acid damage at the start of each of the Bloodfin's turns. If the Bloodfin takes 20 damage or more on a single turn from a creature inside it, the Bloodfin must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate the swallowed creature, which falls prone in a space within 10 feet of the Bloodfin. If the Bloodfin dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 10 feet of movement.