Besil, Dragon in Human Form

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

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

Spellcasting. The Besil is an 18th-level spellcaster. her spellcasting ability is Charisma (spell save DC 24, +16 to hit with spell attacks). 

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, thunderclap

1st level (4 slots): detect magic, disguise self, shield, magic missile

2nd level (3 slots): hold person, Scorching Ray, misty step

3rd level (3 slots): counterspell, fly, fireball

4th level (3 slots): greater invisibility

5th level (3 slots): dominate person, Telekinesis, Teleportation circle

6th level (1 slot): Investiture of Flame

7th level (1 slot): delayed blast fireball

8th level (1 slot): Incendiary Cloud

9th level (1 slot): Wish

Metamagic (5/Day): Besil can twist spells to suit her needs. When casting a spell, she can apply one of the following effects:
Quickened Spell: If the spell has a casting time of 1 action, Besil changes the casting time to 1 bonus action for that casting.
Subtle Spell: Besil changes the spell to cast it without any somatic or verbal components.
Heightened Spell: If the spell forces a creature to make a saving throw to resist its effects, Besil gives one target if the spell disadvantage on its first saving throw made against the spell.

Dagger. Melee or Ranged Weapon Attack: +8 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Change Shape. Besil can magically polymorph into a humanoid or beast that has a challenge rating no higher than 17, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (Besil's choice).

In a new form, Besil retains her true form's alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.