Shapechanger. The dragon can use its bonus action or when the it is about to take damage, the dragon can use its reaction to shapechange, to polymorph into any Huge dragon it knows of that has a challenge rating no higher than its own, or back into its true form. Its statistics are the same in each form, except it gains any different forms of damage adjustments along side of its original damage adjustments, it also gains the movement speeds and traits it doesn’t already have. Additionally, the dragon’s breath weapon is replaced with the breath weapon of its new form, if any, but the saving throw against it is a DC 22 Intelligence saving throw, instead of the original saving throw. The dragon loses these changes when it reverts back to its true form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Ambusher. In the first round of a combat, the dragon has advantage on attack rolls against any creature it has surprised.

Surprise Attack. If the dragon surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 14 (4d6) psychic damage from the attack.

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

Expert Chaser (True Form Only). The dragon can choose to take the dash action as a bonus action.

Reflective Scales (True Form Only). Any time the dragon is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d8. On a 1 to 4, the dragon is affected normally. On a 5 to 7, the dragon is unaffected. On a 8, the dragon is unaffected, and the effect is reflected back at the caster as though it originated from the dragon, turning the caster into the target.

Spider Climb (True Form Only). The dragon can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check. 

Night Flier (True Form Only). The dragon's scales allow it to blend in with the night sky, granting advantage on Stealth rolls made while flying at night.

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: +11 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage plus 4 (1d8) force damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 14 (2d8 + 5) 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 22 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.

False Breath (Recharge 5–6). The dragon exhales illusionary force in a 60-foot cone or a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 24 Intelligence saving throw, taking 55 (10d10) psychic damage on a failed save, or half as much damage on a successful one.

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.

Shift Position (True Form Only). The dragon magically teleports, along with any equipment it is wearing or carrying, up to 50 ft. to an unoccupied space it can see.

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 19 Dexterity saving throw or take 12 (2d6 + 5) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.