Aoxybiont. Mezzoni doesn't require air to live.

Probing Telepathy. If a creature communicates telepathically with Mezzoni, Mezzoni can learn the creature’s greatest desires.

Mind-Bending. Mezzoni's weapon attacks are magical. When he hits with any weapon, the weapon deals an extra 4d8 psychic damage (included in the attack).

Psychiatrist. Mezzoni can tell when a creature is lying to him.

Spellcasting. Mezzoni's spellcasting ability is Wisdom (spell save DC 22). Mezzoni can innately cast the following spells, requiring only verbal components:

At Will: cause fear, charm person, charm monster, clairvoyance, confusion, crown of madness, detect thoughts, mind spike, [spell]summon aberration[/summon]. 

10/day: dominate person, dream, eyebite, mental prison, blindness/deafness

3/day: feeblemind. 

 Axe. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (1d10 + 4) slashing damage plus 20 (4d8) pyschic damage.

Enslave (3/Day). Mezzoni targets one creature it can see within 30 feet of it. The target must succeed on a DC 20 Wisdom saving throw or be magically charmed by Mezzoni until he dies or until it is on a different plane of existence from the target. The charmed target is under Mezzoni's control and can’t take reactions, and Mezzoni and the target can communicate telepathically with each other over any distance.

Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from Mezzoni.

Imperfection. When Mezzoni is first attacked, he can change into his Imperfect Form. This form gives him an advantage on Strength and Constitution rolls. In addition, any creature that can see him must make a Wisdom saving throw (DC 20). On a failed roll, they are frightened. 

However, when in this form, he is unable to cast spells or use his Enslave action. 

Mezzoni can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Mezzoni regains spent legendary actions at the start of his turn.

Detect. Mezzoni makes a Wisdom (Perception) check.

Psychic Drain (Costs 2 Actions). One creature charmed by Mezzoni takes 10 (3d6) psychic damage, and Mezzoni regains hit points equal to the damage the creature takes.