Shapechanger. The fiend can use its action to polymorph into a Small or Medium humanoid, or back into its true form.  Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Spellcasting. Zehir is a 10th level spellcaster. He has access to the wizard, warlock, bard, and cleric spell lists, but casting a cleric level deals 20 radiant damage to him. His spellcasting modifier is +10, and his spell save DC = 18. He can cast the following spells:
At Will: Mage Hand, Lightning Lure, Minor Illusion, Prestidigitation, Toll the Dead, Eldritch Blast, 
5/day: Absorb Elements, Burning Hands, Cure Wounds, Chromatic Orb, Hellish Rebuke, Hex, Ice Knife, Shield, Shield of Faith, Tasha´s Hideous Laughter, Tenser´s Floating Disk.
4/day: Blur, Cloud of Daggers, Darkness, Darkvision, Dragon´s Breath, Enlarge/Reduce, Flaming Sphere, Hold Person, Misty Step, Shadow Blade, Vortex Warp.
4/day: Ashardalon´s Stride, Fireball, Haste, Lightning Bolt, Tidal Wave, Vampiric Touch.
3/day: Death Ward, Ice Storm, Storm Sphere, Vitriolic Sphere.
2/day: Steel Wind Strike, Bigby´s Hand.

Zehir casts a spell, makes a single claw attack, or attacks with Justice twice.

Claw (Fiend Form Only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) slashing damage.

Charm. One humanoid the fiend can see within 30 feet of it must succeed on a DC 15 Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys the fiend's verbal or telepathic commands. If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, the target is immune to this fiend's Charm for the next 24 hours.

The fiend can have only one target charmed at a time. If it charms another, the effect on the previous target ends.

Etherealness. The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa.

Justice. Melee Weapon Attack: +10 to hit, reach ? ft., one target. Hit: 11 (2d8 + 7) magical slashing damage.

Justice. If Zehir didn´t use his action to use Justice, he can use his bonus action to make a single Justice attack.

Hex.
Misty Step.
Shadow Blade.
Dragon´s Breath.

Shield.
Absorb Elements.
Hellish Rebuke.