Thunder dragons are very rare, and their origins are believed to occur from the mating of blue and white dragons, though it is also believed to be the work of storm magic infused into a white dragon's egg. This is based on thunder dragons feral nature, similar to white dragons. 

They typically dwell in windswept mountain ranges or along coastal cliffs, using powerful winds and lightning to hunt their prey. They are some of the most agile dragons, using their speed and dexterity to keep their foes off balance until they fall to the ground below.

A thunder dragon has cloudy eyes, a sleek profile, and spines pointing backwards along its head and body. The scales of a wyrmling thunder dragon are a strong shade of plum, and as the dragon ages its scales begin to lighten, so that by the time it is old, it resembles a white dragon with pale lavender coloring. 

Primal and Deadly. Thunder dragons are ferocious predators that act largely out of wrath and hunger. While they will kill any large animal medium-sized or larger creature that ventures too close to its lair, they prefer only to eat fresh kills, so they often kill simply for sport and territory, and will bring carcasses and drop them at the base of their lair as trophies. A large pile of bones at the base of a sheer cliff is often an indication that a thunder dragon lives above, while the most precious bones are kept as part of the dragon's hoard with any items or treasure found among the bodies. The dragon will remember where each prized bone came from, and will lash out in anger if any are taken, as they are used for both status and invoking fear to anyone that might enter their lair.

Thunder dragons enjoy the feeling of power, especially tormenting creatures before hurtling them to their death.  Fear is the emotion that is most treasured by a thunder dragon, and it may spare the lives of creatures if they can be cowed into obedience. At the first sign of disobedience a thunder dragon will immediately kill even the most trusted servant.  

So few thunder dragons have been seen that it is unknown if they are able to reproduce with other dragons. It is believed that they are too territorial to abide any other dragons near their lairs.

Thunder dragons make their lairs in windswept mountain ranges or along coastal cliffs, often competing with bronze and silver dragons for suitable lairs.  They often use the powerful winds and their lightning breath to push their prey off sheer cliffs to fall to their death.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects:

Hail. Hailstones rain down from the cloud. Each creature under the cloud takes 2d6 bludgeoning damage.

Lightning Bolts. The dragon calls six bolts of lightning from the cloud to strike six creatures or objects of its choice beneath the cloud. A given creature or object can't be struck by more than one bolt. A struck creature must make a Dexterity saving throw. The creature takes 10d6 lightning damage on a failed save, or half as much damage on a successful one. This effect can only be used once per day.

Regional Effects

Thunderstorms rage within 6 miles of a legendary thunder dragon’s lair.  Gusts of wind and rain assail the area under the cloud. The area becomes difficult terrain and is lightly obscured. Ranged weapon attacks in the area are made at disadvantage. The wind and rain count as a severe distraction for the purposes of maintaining concentration on spells - add 5 to the DC of any concentration checks from damage, and each creature must succeed on a DC10 concentration check at the start of their turn in combat. Finally, gusts of strong wind (ranging from 20 to 50 miles per hour) automatically disperse fog, mists, and similar phenomena in the area, whether mundane or magical.

If the dragon dies, changed weather reverts to normal, as described in the spell, and the other effects fade in 1d10 days.