Vorkal, King of all Imps

Shapechanger. Vorkal can use its action to polymorph into a beast form that resembles a rat (speed 20 ft.), a raven (20 ft., fly 60 ft.), or a spider (20 ft., climb 20 ft.), or back into its true form. Its statistics are the same in each form, except for the speed changes noted. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
Devil's Sight. Magical darkness doesn't impede the imp's darkvision.
Magic Resistance. The imp has advantage on saving throws against spells and other magical effects.

Kings Will. Exerting his will at all times. Every imps in a 120 feet radius, must obey the commands of the king. Reguardless of the order, as if they were under the dominate monster spell.

 Innate Spellcasting. Vorkal spellcasting ability is Charisma (spell save DC 16). It can innately cast the following spells, requiring no material components. 
At will: Mage Armor, Prestidigitation, Thaumaturgy, Unseen Servant
3/day each: Hex, Darkness, Suggestion, Shield
1/day: Polymorph

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

Sting (Bite in Beast Form). Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 10 (2d4 + 5) piercing damage, and the target must make on a DC 15 Constitution saving throw, taking 21 (6d6) poison damage on a failed save, or half as much damage on a successful one.
Invisibility. The imp magically turns invisible until it attacks or until its concentration ends (as if concentrating on a spell). Any equipment the imp wears or carries is invisible with it.

Vorkal can take 3 legendary actions, using the options below. It can take only one legendary action at a time and only at the end of another creature’s turn. Vorkal regains spent legendary actions at the start of its turn.

Attack. Vorkal uses his sting (bite in beast form).

Move. Vorkal can move 30 feet.

Invisibility (2 points). Vorkal turns invisible.