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

Storm aura. The dragon emits an aura of rain and heavy wind in a 60ft sphere around it. Creatures that start their turn in the aura take 3 (1d6) thunder damage and have their speed reduced by 5ft.

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 16 (2d10 + 5) piercing damage plus 5 (1d10) lighting damage. 

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 12 (2d6 + 5) slashing damage. 

Squall blast (Recharge 5-6).  The dragon exhales lightning and thunder in a 30-foot cone. Each creature in that cone must make a DC 16 Dexterity saving throw, taking 27 (5d10) lighting damage and being blinded on a failed save, or half as much damage and not blinded on a successful one. Each creature in the cone must make a DC 16 Constitution saving throw, taking 27 (5d10) thunder damage and being deafened on a failed save, or half as much damage and not deafened on a successful one.

Storm aura surge (Recharge 5-6). The dragon emits an aura of rain and heavy wind to surge around it. Creatures that start their turn within 10ft of the dragon take an additional 3 (1d6) thunder damage and must make a DC 16 Strength saving throw or have their movement speed halved. Ranged weapon attacks are made at disadvantage. The surge lasts until the start of the dragon's next turn.