Adult Steel Dragon

 Foremost Expert. On any ability check relating to knowledge, the dragon's bonus is equal to their History bonus. Furthermore, pick one area of specialty knowledge. On any ability check relating to this field, the dragon treats a d20 roll of 9 or lower as a 10.

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

Multiattack. The dragon can use its Mighty 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 plus 4 (1d8) acid damage.

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.

Mighty 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 frightened or charmed (the dragon's choice) 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 Mighty Presence for the next 24 hours.

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

Acid Breath. The dragon exhales acid in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) acid damage on a failed save, or half as much damage on a successful one.

Sleeping Gas. The dragon exhales a sweet smelling gas in a 60-­foot cone that is 10 feet tall. Each creature in that cone must make a DC 18 Constitution saving throw, or be incapacitated for 1 minute, until they take damage, or until someone uses an action to wake them. 

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, the Mighty Presence action, 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.

However if threatened (or simply startled) the dragon can also gain access to its any of its movement speeds, Armor Class and Strength, Dexterity and Constitution scores in this form, but doing so reveals many draconic features (growing scaly wings, unnatural strength, etc).

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 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.