Aral

Magic Resistance. The Aral has advantage on saving throws against spells and other magical effects.

Shapechanger. The Aral 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. The Aral is a 10th level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The Aral has the following Wizard spells prepared: 

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, ray of frost

1st level (4 slots): burning hands, identify, magic missile

2nd level (3 slots): magic weapon, mirror image, misty step, shatter, suggestion

3rd level (3 slots): counterspell, dispel magic, fireball, lightning bolt, sending

4th level (3 slots): ice storm, stoneskin

5th level (2 slots): Bigby’s hand, cone of cold

6th level (1 slot): chain lightning, wall of ice

Multiattack. The Aral makes three attacks, one with its bite and two with its claws.

Bite. (Fiend Form Only). Melee Weapon Attack: +7 to hit, reach 5ft, 11 (2d8+3) piercing damage, and the target must make a Constitution saving throw. On a failure, the target takes 6 (2d6) poison damage and is poisoned for 1 minute. A success results in taking half as much poison damage, and not being poisoned.

Claw (Fiend Form Only). Melee Weapon Attack: +7 to hit, reach 5ft, 9 (2d6+3) slashing damage, and the target is grappled.

Burning Soul (Fiend Form Only). The Aral targets one creature with a soul and forces them to make a Wisdom saving throw (DC 17). On a failed save, the target begins to feel an immense burning sensation within themselves and takes 16 (4d8) fire damage. The creature repeats the save at the beginning of each of their turns, repeating the damage at the end of each turn they fail to save. Otherwise this effect lasts for one minute, or until the Aral is knocked unconscious or dies. On a successful save, the target is immune to this effect for the next 24 hours. The Aral may only target on creature at a time with this effect.