Belskiathen, The Deep One

Amphibious. The Deep One can breathe air and water.

Dreamscent. The Deep One knows the location of every sleeping or unconscious creature within 1 mile of it. Within 120 feet, invisible creatures can't hide from it if they are frightened, charmed, or unconscious. 

Lure of the Abyss. The luminous growth hanging before the Deep One's jaws projects impossible images. A creature that starts its turn within 30 feet of the Deep One and can see the lure must succeed on a DC 18 Wisdom saving throw or have disadvantage on its next attack roll, charmed and be unable to make opportunity attacks until the start of its next turn.

Uncertain Reality. When a creature misses the Deep One with an attack, the Deep One can move up to 10 feet without provoking opportunity attacks as the struck body dissolves into black seawater and the real creature appears nearby.

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

Multiattack. The Deep One makes three attacks: one with its Bite and two with its Claws. It can replace one claw attack with Dream-Eater's Gaze. or can use its entire action to use Devour the Dream 

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) piercing damage plus 7 (2d6) psychic damage. make a DC 18 Constitution saving throw , on a fail the creature is paralysed

Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) slashing damage.

Dream-Eater's Gaze. One creature the Deep One can see within 60 feet must make a DC 18 Wisdom saving throw. On a failure, the target takes 18 (4d8) psychic damage and is frightened of the Deep One until the end of its next turn.

If the target is already Frightened ,Charmed, or Paralysed, it instead takes 27 (6d8) psychic damage, and the Deep One gains temporary hit points equal to half the psychic damage dealt.

Devour the Dream (Recharge 5–6). The Deep One tears open the boundary between waking and nightmare. Each creature of its choice within 30 feet must make a DC 18 Charisma saving throw.

On a failed save, a creature takes 36 (8d8) psychic damage and becomes trapped in a waking nightmare until the end of its next turn. While trapped, its speed is 0, it can't take reactions, and it perceives all other creatures as nightmarish enemies. Upon reaching the affected player in the turn order, they must use there action to attack an allied creature, the Deep One chooses the attack's target from among eligible creatures. after this the effect ends.

On a successful save, the creature takes half damage and suffers no other effect.

False Depths. The Deep One sheds two pieces of its essence in an unoccupied space it can see within 80 feet. These shadows last until the start of the Deep One's next turn.

When a creature targets the Deep One with an attack whilst one or both of its essences are within 60 feet of it, roll a d6. On a 1–3, the attack passes through the illusion and breaks one of the essences 

to destroy an essence you must roll 1d6 on a hit (ac 18) if hit the essence will be destroyed,  as long as these two essences are left alive , the bonus action is able to be used, if both are destroyed during the round before the deep ones turn, it is unable to use the ability for the next hour

This creature has no reaction abilities

The Deep One can take 3 legendary actions, choosing from the options below, at the end of another creature's turn. It regains all spent legendary actions at the start of its turn.

Skitter. The Deep One moves its full movement without provoking opportunity attacks.

Psychic Needle. One creature the Deep One can see within 60 feet must succeed on a DC 18 Intelligence saving throw or take 10 (3d6) psychic damage and be unable to take reactions until the start of its next turn.

Nightmare Made Flesh (Costs 2 Actions). The Deep One chooses a creature it can see within 60 feet and manifests one of that creature's deepest fears beside it. The target must make a DC 18 Wisdom saving throw. On a failure, it takes 18 (4d8) psychic damage and is frightened until the end of its next turn. During that time, the Deep One is invisible to that creature, which instead perceives the manifested nightmare occupying the Deep One's space.

Swallow the Light (Costs 3 Actions). The Deep One extinguishes all nonmagical light within 60 feet. Magical light created by a spell of 3rd level or lower is also dispelled. Until the end of the Deep One's next turn, the area within 30 feet of it is filled with supernatural darkness. The Deep One can see normally through this darkness.