Billy the Axolotl

Slippery Body. Grapples on the axolotl have disadvantage due to it's slimy fish body. 

Amphibious. Billy can breathe air and underwater. 

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

Regeneration. Billy may spend one of its multiattack actions to heal 1d10 + 4 health and regenerate a limb.

Camouflage. Billy has advantage on Dexterity (Stealth) checks made to hide in swampy, underwater and dungeon terrains (provided there is ample moisture present).

Sludge Breath. Billy spits out a torrent of toxic water at an enemy. It deals 12 (1d6+3) poison damage except it doesn't acknowledge immunity to water and deals extra damage to fire elementals. Fire newts have an immunity to this power. It can only be used twice per day. 

Frightful Presence. Each creature of Billy's choice that is within 120 feet of Billy and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 Billy's Frightful Presence for the next 2 turns.

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

Water Form. Billy can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Elemental Nature. An eternal conqueror doesn't require air, food, drink, or sleep.

Siege Monster. Billy does double damage on objects and structures

Multiattack. Billy can use its Frightful Presence or Regeneration. He then can make up to four attacks: one with his bite, one with his tail and two with his claws. 

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 5) slashing damage.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 14 (2d10 + 5) piercing damage plus 8 (2d8) acid damage.

Tail. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 7) bludgeoning damage and the target is grappled (escape DC 14). Until this grapple ends, the target is restrained, Billy can automatically hit the target with its tail.

Frightful Presence. Each creature of Billy's choice that is within 120 feet of Billy and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 Billy's Frightful Presence for the next 2 turns.

Sludge Breath. Billy spits out a torrent of toxic water at an enemy. It deals 12 (1d6+5) poison damage except it doesn't acknowledge immunity to water and deals extra damage to fire elementals. Fire newts have an immunity to this power. It can only be used twice per day. 

Water Vortex: Target player must make a DC 18 Dexterity saving throw. if succeeds then player gets pushed 10-20 feet from original spot. if failed, target player takes 13 (2d10 +2) slashing damage.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Parry. Billy adds 3 to its AC against one melee attack that would hit it. To do so, Billy must see the attacker and be wielding a melee weapon.

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

Detect. Billy makes a Wisdom (Perception) check.

Water whip. Billy makes a water whip. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) slashing damage.

Control Water (Costs 2 Actions). Billy casts control water with the bludgeoning damage of the whirlpool and to objects is increased to 6d8 rather than 2d8 (spell save DC 21). Billy maintains concentration and has advantage on Constitution saving throws to maintain it.