Ancient Comet Dragon

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
Meteoric Wrath. Attacking a prone creature doesn't impose disadvantage on the dragon's ranged attack rolls, and any cold, fire, or thunder damage the dragon deals ignores resistance to those types.
Outgas. The dragon exudes a constant barrier of dust and debris. The area within 30 feet of the dragon is lightly obscured and is difficult terrain for all creatures that it chooses. If the dragon takes radiant damage, additional debris is flung from its body; until the dragon uses its Meteor Breath, a creature affected by this difficult terrain must spend 4 feet of movement for every 1 foot it moves in the area.

Multiattack. The dragon can use its Bow Shock. It then makes three melee attacks: one with its bite and two with its claws. It can use Spit Meteor in place of any melee attack.
Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one
target. Hit: 16 (2d10 + 5) piercing damage.
Claw. Melee Weapon Attack: +12 to hit, reach 5 ft., one
target. Hit: 12 (2d6 + 5) slashing damage.
Tail. Melee Weapon Attack: +12 to hit, reach 30 ft., one
target. Hit: 14 (2d8 + 5) bludgeoning damage plus 9
(2d8) cold damage, and the target can't take reactions
until the end of the dragon's next turn.
Spit Meteor. Ranged Weapon Attack: +16 to hit, range
120/240 ft., one target. Hit: 16 (2d6 + 9) bludgeoning
damage plus 7 (2d6) fire damage.
Bow Shock. The dragon focuses its debris at a point it
chooses within 30 feet of it and then causes the debris
to burst in a 20-foot-radius sphere centered on that
point. Each creature of the dragon's choice in the area
must make a DC 23 Constitution saving throw. On a
failed save, the creature's speed is halved and it has
disadvantage on attack rolls until the end of the
dragon's next turn.
Meteor Breath (Recharge 5-6). The dragon unleashes a barrage of meteors in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 23. Dexterity saving throw, taking 35 (10d6) bludgeoning damage and 35 (10d6) fire damage on a failed save, or half as much damage on a successful one. In addition, the dragon's debris ignites (see Outgas trait). Until the start of the dragon's next turn, whenever a creature enters the area for the first time on a turn or starts its turn there, it takes 5 (1d10) fire damage.

Saving Throws Dex +16, Con +15, Int +11, Wis +10
Skills Acrobatics +16, Athletics +12, Stealth +16
Damage Resistances fire, thunder; bludgeoning,
piercing, and slashing from nonmagical weapons
Damage Immunities cold
Condition Immunities deafened, petrified, prone,
restrained, stunned
Senses truesight 150 ft., passive Perception 13
Languages Celestial, Common, Draconic
Challenge 24 (62,000 XP)

LEGENDARY ACTIONS
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.

METEOR SHOT. Ranged Weapon Attack: +16 to hit, range 120/240 ft., one target. Hit: 16 (2d6 + 9) bludgeoning damage plus 7 (2d6) fire damage.

TWIN TAIL. The dragon makes a tail attack. If the attack misses, it can make another tail attack as part of the same legendary action.

SHOOTING STAR (COSTS 2 ACTIONS). The dragon moves up to twice its speed in a straight line. This movement does not provoke opportunity attacks, and if the dragon is grappled, that condition ends immediately before this movement. Each creature in the line takes 7 (2d6) thunder damage and must make a DC 24 Strength saving throw; a creature under the effects of the dragon's Bow Shock has disadvantage on this saving throw. On a failure, a creature takes another 12 (2d6 + 5) bludgeoning damage and is flung up to 20 feet away from the dragon's path and knocked prone. On a success, the creature takes half as much bludgeoning damage and isn't flung away or knocked prone.