Chardalyn Devastator

Spell Turning. The dragon has advantage on saving throws against any spell that targets only the dragon (not an area). If the dragon's saving throw succeeds and the spell is of 7th level or lower, the spell has no effect on the dragon and instead targets the caster.

Drones. The dragon stores up to ten drones (chardalyn mephits) on its inside. It can open it's belly to release all currently stored drones in unoccupied spaces within 5 ft. of it or to get all the drones within 5 ft. stored within.

Stored drones have full cover within the body of the dragon. The dragon can spend 10 minutes creating new drones and repairing damaged ones on its inside. In the beginning of the process, the dragon takes 10 damage for each newly created drone and 5 for each damaged. During creation of the drones, the dragon must be on ground, it has disadvantage of Wisdom (Perception) ability checks and becomes paralyzed (regardless of its usual immunity). It can stop the process at any moment but all the progress and hp are lost. It can make intelligent choices about when and how many drones to create.

Immutable Form. The dragon is immune to any spell or effect that would alter its form.
Magic Resistance. The dragon has advantage on saving throws against spells and other magical effects.

Siege Monster. The dragon deals double damage to objects and structures.

Temperature Meter. Any time the dragon takes fire or cold damage, its inner temperature changes. Mark the damage it has taken, fire damage is added and cold damage is subtracted. When the meter hits +30, the dragon is Overheated, when the meter hits -20, the dragon is Frozen.

Overheated. The insides of the dragon explode and release a massive amount of energy. The dragon and every creature within 20 feet of it takes 35 (10d6) fire damage (creatures other than the dragon can halve the damage by a successful DC 16 Dexterity saving throw) that does not count towards the temperature meter. It's AC is reduced by 1d4. The AC can be reduced in this way to a minimum of 16.

The dragon can use Quick repairs legendary action once per hour to regain 1 AC. The temperature meter is reset to 0.

Frozen. The dragon is under nonmagical effect identical to the Slow spell [Speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn.]

As long as it stays under this effect, it also gains vulnerability to bludgeoning, piercing,
and slashing damage. It makes a DC 16 Constitution saving throw at the end of each of its turns, it has advantage on this saving throw if it took any fire damage since its last turn. On a successful save, the effect ends for it. The temperature meter is reset to 0 when the
effect starts.

Weak spots. The dragon has several weak spots that can be targeted, dealing additional damage dice as specificed. 

Inside. The fine machinery inside of the dragon is especially vulnerable to damage. The spot can be hit only when the dragon takes the Release Drones action by attacks or
AoE spell targeted into this space. Any hit here is automatically critical and thunder damage is maximum possible. If the damage taken by this spot hits 40 point
cumulatively, the dragon loses its ability to produce new drones and repair them.

The AC for the inside of the dragon is 22. Damage to the inside of the dragon deals an additional set of damage dice. 

Mouth. This spot can be targeted only in the round that the dragon uses its Breath Weapon action by attacks or AoE spell targeted into this space. Any attack on this spot is made with half cover. Any hit into the fine machinery of the dragons mouth is always critical and thunder damage makes maximum damage. When the cumulative damage to the mouth reaches 25, the dragon loses the Laser Spree option for its breath weapon. When the cumulative damage to the mouth reaches 60, the dragon loses the Cone option for its breath weapon.

The AC for the mouth is 22 regardless of its current AC. Damage to the dragon's mouth deals an additional 2 damage dice of its primary type.

Wings.
Any attack to the wings is made with disadvantage, but the AC is 16 regardless of the current AC. Every time the cumulative damage to the wings reaches 30 hp mark, the dragon's flying speed is reduced by 5 ft. Its flying speed cannot be reduced below 60ft in this way.

Eyes.
Attacks against this week spot are made with 3/4 cover. Every hit is automatically critical. Upon every hit into this spot, the dragon makes a Constitution saving throw, DC is calculated as for Concentration checks and gains cumulative +1 boost every time the dragon rolls the save. Upon a fail, it becomes blinded for a minute.

Multiattack. The dragon uses its Malevolent Presence. It then makes three attacks: two with its claws and one with its tail. If the dragon isn't flying, it can also make one attack with its wings. Instead of one of these attacks, it can use the Release Drones action.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 21 (2d6 + 7) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) bludgeoning damage.

Wings. The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 16 Strength saving throw or take 12 (1d6 + 7) bludgeoning damage,
be pushed 15 ft. away from the dragon and knocked prone. On a successful save, the creature takes half the damage and is not pushed and knocked prone.

Malevolent Presence. Any creature with an Intelligence of 4 or more that is within 30 feet of the dragon must succeed on a DC 16 Wisdom saving throw or be frightened by it for 1 minute. A creature can repeat the saving throw at the end of each of its 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 Malevolent Presence for the next 24 hours. If the saving throw fails by 5 or more, this creature is paralyzed whilst the dragon is in it's line of sight.

Release Drones. The dragon opens the space in its belly to release all stored drones. It can also use this action to store all drones within 5 ft. of it.

Radiant Breath (Recharge 5–6). The roll for the recharge is made at the end of the dragons turn. Successful recharge is visible. The dragon uses one of the option below if available.

Laser spree. The dragon makes ten ranged weapon attacks, +11 to hit, range 120 ft., one target. Hit: 7 (2d6) radiant damage.

Cone. The dragon exhales radiant energy in a 60-foot cone. Each creature in that area must make a DC16 Constitution saving throw, taking 31 (7d8) radiant damage on a failed save, or half as much damage on a successful one.

Line. The dragon exhales a ray of radiant energy in a 120-foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 45 (10d8) radiant damage on a failed save, or half as much damage on a successful one.

Leave me. When on land, the dragon can take a reaction when hit by a melee attack to use its Wings action.

Disruptive roar. When the dragon perceives someone casting a spell with a verbal component within 60 ft. from it, it can try to disrupt the casting with a mighty roar. It must make a Constitution ability check, DC is 10 + level of the spell being cast. It cannot take this reaction when Silenced.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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.

Laser Bolt. The dragon shoots two bolts of energy from its mouth. It makes two ranged weapon attacks, +11 to hit, range 120 ft., hit: 7 (2d6) radiant damage. It can take this action only if the Laser Spree option for its breath is available (recharged breath not required).

Quick Repairs. The dragon takes 10 points of damage to receive one of the following effects: it ends one condition affecting it; it regains 1 AC lost by Overheated (can be used once per hour even outside of combat).

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 actions). The dragon makes its Wings action and then moves up to half its speed.

Drone attack (Costs 3 actions). The dragon roars to command all its drones. All drones outside of the dragon within 300 ft. can take reaction to move up their speed and make one attack. This legendary action requires the dragon to be able to make sound and the drones to hear it.