Ustalis

Regeneration. The Ustalis heals 4d8 hit points at the beginning of its turn unless it has taken 20 fire damage in the previous or current round.

Water form. If the Ustalis is in water, as a bonus action it change into water form. In this form, any creature that enters in the spaces it occupies take 2d10 cold damage, it loses it walking speed, its swimming is doubled, any creature that attacks it as disadvantage on attack rolls, and Ustalis heals half of the damage it took in its normal state. If the Ustalis touches land, it becomes its land state.

Amphibious. The Ustalis can breath air and water.

Spikey shell. If a creature attacks the Ustalis and misses, the creature takes 4d4 piercing damage and 2d6 cold damage.

Legendary resistance (3/day). If the Ustalis fails a saving throw, it can choose to succeed it instead.

Darkening Aura. The area up to 50 ft away from the Ustalis is considered magical darkness can only vanishes when the Ustalis dies.

Magical weaponry. The Ustalis’ attacks are magical weapon.

Swallow. The Ustalis does a bite attack against a medium or smaller creature it as grappled. If the bite lands, the creature takes the bite damage, it becomes restrained and blinded, it is no longer grappled, it takes 5d12 acid + 5d6 cold damage, and if the Ustalis takes 65 damage in one turn, it must succeed a DC 18 CON save or regurgitate the the creature 5 ft in front of it. If the Ustalis perishes the swallowed creature can cut it self out of the stomach of the Ustalis.

Multiattack. The Ustalis can make 1 bite attack, 2 tail sting attacks, and 2 slash attack.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 4d12 + 9 piercing damage and the target must succeed a DC 16 STR save or become grappled. 

Tail sting. Melee Weapon Attack: +4 to hit, reach 25 ft., 1 target. Hit: 2d20 + 1 piercing damage and 5d6 poison damage, and the target must make a DC 16 CON save or become poisoned for 11d6 rounds. 

Slash. Melee Weapon Attack: +5 to hit, reach 15 ft., 1 target. Hit: 2d20 + 1 slashing damage, and the target must make a DC 17 STR save or be knocked prone and flung 20 ft away.

The Ustalis can make 4 legendary actions and it can use them at the beginning of last turn in the initiative.

Explosive dash (Costs 2 Actions). The Ustalis moves its movement in one direction, any creature the Ustalis runs over must succeed a DC 20 DEX save or take 2d10 bludgeoning + 4d10 fire damage and are knocked prone.

 Freezing roar. The Ustalis emits a roar (50 ft dome around it), any creature in that dome must make DC 19 CON save, on a fail the creature takes 3d12 cold damage, becomes deafened and stunned.

Shapechange. The Ustalis transforms into a creature of equal CR and gets all their abilities plus its own abilities, for 3d6 + 1 rounds.

Evaporate and condense. The Ustalis evaporates for 1 round, on the next round it moves to an unoccupied space it can see up to 120 ft away.