Living Shadow. While in dim light or darkness, the dragon has resistance to damage that isn'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.   Customizing Dragons
You can customize any dragon's stat block to reflect the dragon's unique character. Minor changes such as those below are easy to make and have no impact on a dragon's challenge rating.   Languages. Most dragons prefer to speak Draconic but learn Common for dealing with allies and minions. But given their high Intelligence and long life span, dragons can easily learn additional languages. You can add languages to a dragon's stat block.   Skills. Most dragons are proficient in the Perception and Stealth skills, and many dragons have additional skill proficiencies. As with languages, you can customize a dragon's skill list (even doubling their proficiency bonus with certain skills) to reflect particular interests and activities. You can also give a dragon tool proficiencies, particularly if the dragon spends time in Humanoid form.   Other Traits and Actions. You can borrow traits and actions from other monsters to add unique flavor to a dragon. Consider these examples:   Damage Absorption. You might decide that this dragon is not only unharmed by fire damage, but actually healed by it.
Whenever the dragon is subjected to fire damage, it takes no damage and instead regains a number of hit points equal to the fire damage dealt.   Flyby. The dragon is an agile flier, quick to fly out of enemies' reach.
The dragon doesn't provoke an opportunity attack when it flies out of an enemy's reach.   Mimicry. Impersonating characters or their allies could be a fun trick for a crafty dragon.
The dragon can mimic any sounds it has heard, including voices. A creature that hears the sounds can tell they are imitations with a successful DC 12 Wisdom (Insight) check.   Rejuvenation. You might decide that dragons in your campaign, being an essential part of the Material Plane, are nearly impossible to destroy. A dragon's life essence might be preserved in the egg from which it first emerged, in its hoard, or in a cavernous hall at the center of the world, just as a lich's essence is hidden in a phylactery.
If it has an essence-preserving object, a destroyed dragon gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the object.   Special Senses. Most dragons have blindsight and darkvision. You might upgrade blindsight to truesight, or you could give a dragon with a burrowing speed tremorsense.   Variant: Dragons as Innate Spellcasters
Dragons are innately magical creatures that can master a few spells as they age, using this variant.   A young or older dragon can innately cast a number of spells equal to its Charisma modifier. Each spell can be cast once per day, requiring no material components, and the spell's level can be no higher than one-third the dragon's challenge rating (rounded down). The dragon's bonus to hit with spell attacks is equal to its proficiency bonus + its Charisma bonus. The dragon's spell save DC equals 8 + its proficiency bonus + its Charisma modifier.   This dragon can innately cast four spells, once per day each, requiring no material components. Each spell's level can be no higher than 4th. The dragon's spell save DC is DC 17, and it has +9 to hit with spell attacks. See the spell page for a list of spells the dragon is capable of casting. A selection of examples are shown below:   fireball
scorching ray
haste
erupting earth
Aganazzar's scorcher