Adult Onyx Dragon

Telepathy. The dragon can telepathically communicate with any creature within 60 feet of it.

Magic Cunning. The dragon has advantage on saving throws against spells and other magical effects.

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

Shielded Mind. The Dragon is immune to scrying and to any effect that would sense its emotions, read its thoughts, or detect its location.

Change Shape. The dragon magically poly-morphs 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, proficienties, 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.

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

Multi-Attack. The dragon can use its Divine Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage.The target must succeed on a DC 14 constitution saving throw or be poisoned for 24 hours Until this poison ends, the target is unconscious. Another creature can use an action to shake the target awake

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Divine 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 16 Wisdom saving throw or become Stunned 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 Divine Presence for the next 24 hours.

Force Breath. The dragon exhales pure magical force in an 60-foot line that is 15 feet wide. Each creature in that line must make a DC 19 Intelligence saving throw, taking 66 (12d10) force damage on a failed save, or half as much damage on a successful one.

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 17 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.

Invisibility. The dragon turns invisible for 1 round, as if using the Invisibility spell. 

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 20 Dexterity saving throw or take 12 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.