Ancient Purple Dragon

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

Breath Control. The dragon has exceptional control over its ability to breathe searing plasma. The Plasma Breath and Plasma Spear actions both use the dragon’s shored up plasma reserve, so if one ability is used, it must recharge before the other can be used.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) fire damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Frightful Presence. Each creature of the dragon’s choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened 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 Frightful Presence for the next 24 hours.

Plasma Breath (Recharge 5-6).

The dragon exhales crackling energy in a 90-foot cone, or a 180-foot line. Each creature in that area must make a DC 25 Dexterity saving throw, taking 98 (28d6) fire damage on a failed save, or half as much damage on a successful one. Fire damage dealt by the dragon’s breath bypasses fire resistance, but not immunity. All creatures within 60 feet of the dragon that can see it must succeed on a DC 25 Constitution saving throw or be blinded for 1d4 rounds.

Plasma Spear (Recharge 5-6).Ranged Spell Attack: +18 to hit, reach 600/1,200 ft., one creature.Hit: 98 (28d6) fire damage. Fire damage dealt by the dragon’s breath bypasses fire resistance, but not immunity. This attack ignores any cover bonus to AC benefiting the target except for cover bonuses granted by objects made of adamantine.  The target must succeed on a DC 25 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction is permanent unless undone with magic such as the Greater Restoration spell. On a critical hit, this attack severs one of the target’s limbs, if it has any. 

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 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Command. The dragon bellows a command to one of its allies within 300 feet. The chosen creature can immediately take one action.