Adult Mithral Shadow Dragon

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

Living Shadow: While in dim light or darkness, the dragon has resistance to damage that ins't force psychic or radiant.

Shadow Stealth. While in dim light or darkness, the dragon can take the hide action as a bonus action.

Sunlight Sensitivity. While in sunlight, the dragon has disadvantage on attack rolls, as well as on Wisdom (perception) checks that rely on sight.

Innate Spellcasting. The dragons innate spell casting ability is Intelligence (Spell save DC18 +10 to hit will spell attacks). It can innately cast the following spells, requiring no material components.
At will: tongues
5/day each: dispel magic, enhance ability

Spellcasting. The dragon is a 6th-level spellcaster, It's spellcasting ability is intelligence (spell save DC 18 +10 to hit spell attacks). It requires no material components to cast its spells, the dragon has the following wizard spells prepared.
Cantrips (at will): mage hand, message, minor illusion, prestidigitation
1st level (4 slots): cause fear, charm person, magic missile, unseen servant
2nd level (3 slots): dragons breath, darkness, web
3rd level (3 slots): bestow curse, thunder step, vampiric touch 

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 5 (1d10) necrotic damage.

Claw. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 8) slashing damage, and hte target loses 4 hit points from bleeding at the start of each of its turns for the next six rounds unless it receives magical healing. Bleeding damage is cumulative; the target loses 4 hit points per round for each bleeding wound its taken from a mithral shadow dragon's claws.

Tail. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 16 (2d8 + 8) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Shadow Breath. The dragon exhales darkness in a 60- foot cone. Each creature in that line must make a DC 18 Dexterity saving throw, taking 66 (12d10) necrotic damage on a failed save, or half as much damage on a successful one. A humanoid reduced to 0 hit points by this damage dies, and an undead shadow rises from its corpse and acts immediately after the dragon in the initiative count. the shadow is under the dragon's control.

Sleep Breath. The dragon exhales sleep gas in a 60-foot cone. Each creature in that area must succeed on a DC 18 constitution saving throw or fall unconscious for 10 minutes. This efect ends for a creature if the creature takes damage or someone uses an action to wake it.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, 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 (the dragon's choice).

In a new form, the dragon retains its 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.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 21 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.