Belina the Sea Hag

Amphibious. Belina can breathe air and water.

Horrific Appearance. Any humanoid that starts its turn within 30 feet of Belina and can see the Belina's true form must make a DC 11 Wisdom saving throw. On a failed save, the creature is frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the hag is within line of sight, 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 the hag's Horrific Appearance for the next 24 hours.

Unless the target is surprised or the revelation of the Belina's true form is sudden, the target can avert its eyes and avoid making the initial saving throw. Until the start of its next turn, a creature that averts its eyes has disadvantage on attack rolls against the hag.

Belina's Form. Belina's form is unstable and is imbued with a permanent Mirror Image effect. though it isn't visible unless you make an 18 perception check or a 17 Investigation check.

Legendary Resistance (4/Day). If the tarrasque fails a saving throw, it can choose to succeed instead.

Multiattack.  Belina can make 2 claw swipes 

Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 +4) slashing damage.

Melting Glare (recharge 3-4 rounds). Belina targets one creature she can see within 45 feet of her. If the target can see the hag, it must succeed on a DC 18 Wisdom saving throw against this magic or take 14(4d6) Psychic damage. If the target has an negative modifier to wisdom and Fails the saving throw then the Psychic damage they take 3 times the damage

Illusory Appearance. Belina covers herself and anything she is wearing or carrying with a magical illusion that makes her look like an ugly creature of her general size and humanoid shape. The effect ends if the hag takes a bonus action to end it or if she dies.

The changes wrought by this effect fail to hold up to physical inspection. For example, the hag could appear to have no claws, but someone touching her hand might feel the claws. Otherwise, a creature must take an action to visually inspect the illusion and succeed on a DC 16 Intelligence (Investigation) check to discern that the hag is disguised.

Belina has 1 legendary action. She regains a use of it at the end of her turn. 

Belina's Scratch. Belina scratches at a creature within 5ft of her with a +4 to hit, This attack deals 1d4 slashing damage, and 2d6 necrotic damage.

Water bolt. Belina conjures water into her hand and [Tooltip Not Found] with a +7 to hit

Shell Armor. Belina can  pull shell fragments to her giving her a +2 to her Ac. This lasts for until her next turn, if she is hit with it up she looses the +2 to Ac, but their person who hit takes 1d8 Piercing damage.