Avolakia

Shapechanger. An avolakia can use its action to polymorph into a Small or Medium humanoid it has seen, or back into its true form. While in humanoid form, the avolakia can use its vocal cords to speak any language it knows, rather than just its native language. Its statistics, other than its size, 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.

Innate Spellcasting. The avolakia's spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). The avolakia can innately cast the following spells, requiring no material components: 

At will: chill touch, mage hand, vampiric touch

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

Poisonous Slime (Avolakia Form only). A creature that touches the avolakia or hits it with a melee attack while within 5 feet of it takes 9 (2d8) poison damage.

Regeneration. The avolakia regains 10 hit points at the start of its turn. If the avolakia takes acid or lightning damage, this trait doesn't function at the start of the avolakia's next turn. The avolakia dies only if it starts the turn with 0 hit points and doesn't regenerate.

Multiattack. The avolakia makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus 7 (2d6) poison damage. 

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.