Amphibious. Abaia can breathe both air and water. 

Intimidating Entry. At the start of each of Abaia's turns, each creature that can see her must make a DC 22 Wisdom saving throw or become Frightened for 1 minute. A Frightened creature can repeat the save at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw succeeds or the effect ends for it, it is immune to Abaia's Intimidating Entry for the next 24 hours. 

Magic Weapons. Abaia's weapon attacks are magical.

Mucosal Epidemis. Abaia cannot be grappled or Restrained by physical means. She also gains advantage on saving throws against spells and other magical effects that would cause her to be grappled or Restrained. 

Legendary Resistance(4/day). If Abaia fails a saving throw, she can choose to succeed instead. 

Siege Monster. Abaia deals double damage to objects, structures, and vehicles. 

Multiattack: Abaia makes three attacks, two with her Tail and one with her Bite. Her Bite attack can be replaced with Swallow if she is grappling a creature.

Bite: Melee weapon attack, +17 to hit, reach 5ft, one target. Hit 28(5d6+10) piercing damage. If the target is large or smaller, it is grappled(escape DC 20). 

Tail: Melee weapon attack, +17 to hit, reach 45ft, one target. Hit 24(4d6+10) bludgeoning damage and the creature must succeed on a DC 20 Dexterity saving throw or become grappled(escape DC 20). While grappled in this way, the target is Restrained.

Swallow: Abaia makes a Bite attack against one creature she is grappling. If the attack hits, the target is swallowed and the grapple ends. A swallowed creature is Blinded and Restrained, has total cover from attacks and other effects outside Abaia, and takes 49(14d6) acid damage at the start of each of Abaia's turns. 

If Abaia takes 40 or more damage in a single turn from a swallowed creature, she must succeed on a DC 16 Constitution saving throw or regurgitate all swallowed creatures, which fall Prone in an unoccupied space within 5ft of Abaia. If she dies, a swallowed creature is no longer Restrained and can escape the corpse using 20ft of movement, exiting Prone.  

Lightning Surge(Recharge 5-6): Abaia releases a blast of electrical energy from her body. Each creature within 60ft of her must make a DC 24 Dexterity saving throw, taking 52(8d12) lightning damage on a failed save, and half as much on a successful one. 

Abaia can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Abaia regains spent legendary actions at the start of her turn. 

Tail: Abaia makes one Tail attack.

Move: Abaia moves up to half her speed. 

Upchuck(Cost 3 Actions): Abaia spews forth the contents of her stomach in a 60ft cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 26(4d12) acid damage on a failed save, and half as much on a successful one. All swallowed creatures are regurgitated as well, and they take 52(8d12) acid damage and fall Prone in an unoccupied space within 60ft of Abaia.