Crodelva

Amphibious. The crodelva can breathe air and water.

Underwater Camouflage. The crodelva has advantage on all Stealth checks when submerged in water.

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

Multiattack. The crodelva makes two attacks: one with its mouth (Frost Bite or Death Roll) and one with its tail.

Frost Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (1d10 + 5) piercing damage, and the target is grappled (escape DC 14). Until this grapple ends, the crodelva  can't bite another target. The target must also make a DC 14 Constitution saving throw, taking 8 (2d8) cold damage on a failed save, or half as much damage on a successful one. If the target failed the Constitution their speed is reduced by 5ft for 1 hour. This effect can be removed by any ability, item, or spell that would remove the poisoned condition.

Death Roll. Grappled Melee Attack: +5 to hit, reach 5 ft., one target. Hit: 21 (3d10 + 5) piercing damage, and the target is restrained (escape DC 16). Until this restraint ends, the crodelva  can't bite another target.

Tail. Melee Weapon Attack: +5 to hit, reach 10 ft., one target not grappled by the crodelva. Hit: 9 (1d8 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked prone.

Brine Bile. When targeted by a melee attack within 10ft, the crodelva can use it's reaction to spray caustic bile from its mouth at the attacker. The target must make a DC14 Dexterity saving throw. If the target fails they become blinded for 1 min, and continue their melee at disadvantage. This ability can only be used if the crodelva is not currently grappling or restraining a creature in it's mouth.