Amvicius, the Thirsty Serpent

Amphibious. Amvicius can breathe air and water.

Legendary Resistance (3/Day). If Amvicius fails a saving throw, it can choose to succeed instead. 

Magic Resistance. Amvicius has advantage on saving throws against spells and other magical effects.  

Change Shape. At will, Amvicius can morph into its land-based form, The Faceless Ape. In this new form, Amvicius retains only its alignment. All other statistics, like ability scores, proficiencies, and actions, are prone to change. 

Multiattack. Amvicius can use its Intimidating Glare. It then makes three attacks but cannot make more than one Bite and one Tail attack; or it uses Leaping Wave. 

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage.

Tentacle. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 15 (1d12 + 8) slashing or bludgeoning damage.

Tail. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage. Instead of doing damage, Amvicius can instead grapple the target (escape DC 21). The target is pulled into Amvicius' space while grappled in this manner. 

Intimidating Glare. One creature of Amvicius' choice that is within 120 feet of Amvicius and aware of it must succeed on a DC 19 Wisdom saving throw or become paralyzed for 1 minute. A creature can repeat the saving throw at the start of each of its turn, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Amvicius' Intimidating Glare for the next 24 hours.

Leaping Wave. Amvicius stops grappling any creatures, if any. Then it leaps high out of the water and lands with a massive 30-foot radius splash. All Large or smaller creatures within this radius are automatically pushed 30 feet away from Amvicius. They must also make a Strength saving throw or take 14 (4d6) bludgeoning damage and be knocked prone. 

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

Swim. Amvicius moves up to half his speed. 

Tail Attack. Amvicius makes a tail attack.

Constrict (Costs 2 Actions). Amvicius squeezes a creature grappled by it. Amvicius makes a tail attack: if it hits, the target takes 30 (4d10 + 8) bludgeoning damage, becomes restrained (escape DC 21), and begins to drown as it's pulled under the water. 

Whirlpool (Costs 3 Actions). Amvicius quickly spirals around in the water, creating a 30-foot radius circle of swirling water around it. The swirling radius becomes difficult terrain. This lasts for 1 minute. 
During this minute, creatures who enter the radius must make a Strength saving throw. On a failure, they are flung counter-clockwise 60 feet and plunge into the water, taking 21 (3d8 + 8) bludgeoning damage. On a success, they can freely enter the radius. They must repeat this saving throw at the end of each of their turns.