Horror of the Depths: In this creatures true form any creature rolling initiative must make a INT save dc 14 or have disadvantage on every roll in the fight (out of fear)
Monsterous Speed: This creature gets 2 actions and 2 bonus actions per turn
Abyssal Lungs: this creature is able to breath under water and on land
Old Ones Vision: This creature has 200 Ft of dark vision and cant be blinded, this creature's dark vision doubles under water
Adaptive Limbs: This creature has double its walking speed as its swimming speed
Banish Immune: This creature cant be banished or imprisoned anywhere with any type of magic Not even Wish can be used to banish/imprison this creature

Multi-Attack: This creature is able to use Claws, Tail Swipe and Bite all as one attack
Claws: +6 to hit
Range: Melee
Description: target takes 6d8 piercing damage this iqnores Crit Resistance and piercing resistance and acts as if crit immunity and piercing immunity are just resistances for damaging purposes the target must make a CON save DC 14 or take another 6d8 bleeding damage
Tail Swipe: + 6 to hit
Range: Melee
Description: Target takes 6d6 Crushing damage and must make a STR save DC 16 or be shoved back 20 ft if they hit a wall they take another 3d6 Bludgeoning damage
Bite: + 8 to hit 
Range: Melee
Description: The target takes 5d12 slashing damage and must make a CON save DC 14 or take another 4d8 Necrotic Damage
Breath Weapon: + 4 to hit
Range: 60 Ft
Description: The Target takes 6d8 Cold Damage and another 4d6 Fire Damage After Words the target must make a CON save DC 14 or take another 6d8 Cold Damage and Another 4d6 Fire Damage

This Will be used for all of this creatures spells:
This creature has 6 levels in warlock, Wizard, and sorcerer with no sub class and has 6 spell slots of each level up to 6th that it can use as it pleases
This creature can have up to 8 spells prepared from any of these classes and can use any sub class spells as well
This Creature can also cast True Polymorph and Polymorph for Free an Infinite amount of times

Easy Prey: if a creature gets in melee range of it this creature gets to attack it using a reaction using any of its normal attacks (not spells)
Spell Reversal: if a creature within range tries to target this creature with a spell below 6th level this creature is able to use a reaction to make the target of the spell the caster

(all Legendary Actions for this creature cost 1 action/bonus action depending on choice)
Roar of the Depths: All creatures within a 80 ft radius of the caster must make a WIS save DC 16 or be intimidated and have advantage on all attacks made against them This has 2 uses
Wrath of the Depths: This creature goes on a blood thirsty rampage attacking all creatures within 30 ft of it this acts as a unique attack with the following stats
+ 6 to hit
On hit target takes 12d12 Piercing Damage and is knocked Prone if they survive if they are put on deaths door they instantly have 1 death fail added to their counter
After this attack is done this creature takes 4d12 damage as it calms down from its rage This has one use and can only be used at below half hp

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.