Adult Purple Dragon

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Darkness (level 2 version).

Rend. Melee Attack Roll: +11, reach 10 ft. Hit: 15 (2d8 + 6) Slashing damage plus 7 (2d6) Radiant damage.  

Plasma Breath (Recharge 3–4). Dex Saving Throw: DC 17, each creature in a 30-foot Line. Failure: 70 (20d6) Radiant damage. Success: Half damage.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 17):

At Will: Detect Magic, Darkness (level 2 version)
1/Day: invisibility

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Umbral Light. Dex Saving Throw: DC 17, each creature in a 20-foot-radius Sphere centered on the dragon. Failure: 7 (2d6) Radiant damage, and the target is blinded the end of its next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.  

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.