legendary resistance (3/day).  If the dragon fails a saving throw, it can choose to succeed instead

Innate Spellcasting.  the dragons innate spellcasting is int. (spell save DC 18, +10 to hit with spell attacks) they can cast the following spells, requiring no material components 
    At will: shocking gasp, thunderclap, lighting lure, dancing light
  4/day: thunderwave, Witch bolt, Thunderous smite, fog cloud
  3/day: healing spirit,gust of wind, calm emotions
  2/day: thunder step , tiny servant 
 1/day:storm sphere

Multiattack.  The dragon can use its frightful presence. It then makes 3 attacks one with it bite and two with it claws

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft, one target. hit  9(2d8) piercing damage

claw. Melee Weapon Attack: +5 to hit, reach 5 ft, one target hit 7(2d6) slashing damage

Tail.  Melee Weapon Attack :+5 to hit, reach 5 ft, one target hit 7(2d6) bludgeoning damage

Frightful Presence. Each creature of the dragon's choice that is with in 120 feet  of the dragon and aware of it must succeed on a DC 16 wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of it 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 

lighting breath(5-6)  the dragon exhales lighting in a 40 ft cone . Each creature in it most make a dc 11 dexterity saving throw, taking 49 (11d8) lighting damage on a failed save or half as much 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 regain 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 13 dexterity saving throw or take 7 (2d6) bludgeoning damage and be knocked prone. the dragon can then fly up to half its speed.