Bazelgeuse

Superheated Enrage (Recharges after a Short or Long Rest). If the dragon would be reduced to half its total hit points, all creatures within 10-feet of the dragon have to make a DC 21 Dexterity saving throw or take 10 (3d6) fire damage and be knocked prone. The dragon then gains 200 temporary hit points, it recharges its Bombing Run, Breath Weapon, and Stunning Roar. Additionally, as long as it has temporary hit points, the dragon's Explosive Scales immediately denotate when they hit the ground, and the dragon can now use the options in the “Mythic Actions” section until its temporary hit points. Award a party an additional 18,000 XP (36,000 XP total) for defeating the dragon after its Metallic Awakening activates.

Explosive Scale. The dragon sheds explosive scales through various means, these scales stay on the ground until they explode. The scales explode at the start of the dragon's turn or if they take fire damage and be knocked prone. The scales explode in a 5-foot radius. Each creature in that area must make a DC 21 Dexterity saving throw, or take 10 (3d6) fire damage on a failed save. A creature that touches the dragon or hits it with a melee attack while within 5 feet of it, causes it to drop an Explosive Scale in a unoccupied space adjacent to the attacker.

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

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its headbutt and two with its slam attack.

Headbutt. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage. Hit or Miss: The dragon drops an Explosive Scale within 5 feet of the targeted creature.

Slam. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 16 (2d6 + 9) bludgeoning damage.  Hit or Miss: The dragon drops an Explosive Scale within 5 feet of the targeted creature.

Tail. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 17 (2d8 + 9) bludgeoning damage.  Hit or Miss: The dragon drops an Explosive Scale within 5 feet of the targeted creature.

Bombing Run (Recharge 5-6). The dragon can move up to double its speed until the end of the turn. Additionally for every 15 feet the dragon moves, it drops an Explosive Scale in a unoccupied space it moved through.  

Ember Breath (Recharge 6). The dragon exhales fire in a 15-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one. If any Explosive Scales are within the fire breath's effected area they detonate. 

Stunning Roar (Recharges after a Short or Long Rest). Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Constitution saving throw or become Stunned 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. 

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.

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 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Dive Bomb (Costs 3 Actions). The dragon moves up to twice its speed in a straight line through other creature’s space, that creature must succeed on a DC 17 Dexterity saving throw or take 9 (2d8) bludgeoning damage, 9 (2d8) fire damage and be knocked prone. A creature that succeeds on its saving throw takes half as much damage and isn’t knocked prone.

If the dragon’s Superheated Enraged trait has activated in the last hour, it can use the options below as legendary actions.

Body Slam (Costs 2 Actions). The dragon makes two Slam attacks.

Charge (Costs 2 Actions). The beast moves up to its speed without provoking opportunity attacks, then makes one Headbutt attack.