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

Spellcasting. The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

At will: Detect Magic, Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell), Thunderwave

1/day: Legend Lore (as an action)

Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of (A) Time-Warping Breath or (B) Spellcasting to cast Thunderwave.

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

Ruinous Breath (Recharge 5–6). Constitution Saving Throw: DC 18, each creature in a 60-foot Cone. Failure: 63 (14d8) Necrotic damage. Success: Half damage.

Time-Warping Breath. Wisdom Saving Throw: DC 18, each creature that isn't currently affected by this breath in a 60-foot Cone. Failure: The target's Speed is halved, it can't take Reactions, and it can take either an action or a Bonus Action on its turn, not both. It repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically.

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

Shattering Wave. The dragon uses Spellcasting to cast Thunderwave.

Unearth Ruins. The dragon magically raises broken, ancient ruins in a 20-foot-radius, 60-foot-high Cylinder centered on a point it can see within 120 feet. Ground in the Cylinder becomes Difficult Terrain. Each creature in the Cylinder when it appears is subjected to the following effect. Strength Saving Throw: DC 18. Failure: 5 (2d4) Bludgeoning damage, and the target has the Prone condition. Failure or Success: The dragon can't take this action again until the start of its next turn.