Argrichi

Amphibious. The Argrichi can breath both air and water.

Slumber of the Ancients.  If the Argrichi would be killed by taking damage, it instead falls unconscious for 2d100 years instead, unless the triggering damage is fire or lightning.

Multiattack. the Argrichi makes two bite attacks, or one bite attack and one claw attack.  It can use its Swallow instead of one of its bites.

Bite. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) piercing damage. 

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 16 (3d8 + 3) slashing damage. 

Swallow. The Argrichi makes one bite attack against a Medium or smaller target within 5 feet of it. If the attack hits, the target is also swallowed. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the Argrichi, and it takes 3 (1d6) acid damage at the start of each of the Argrichi's turns. An Argrichi can have only one creature swallowed at a time, and while it has a creature swallowed it becomes Large.

If the Argrichi takes 10 damage or more on a single turn from the swallowed creature, the Argrichi must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the Argrichi. If the Argrichi dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.