Ancient Red Dragon Chosen of Tiamat

Amphibious. The dragon can breathe air and water.

Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes four Rend attacks. It can replace one attack with a use of Spellcasting to cast (A) Mind Spike (level 5 version), (B) Shatter (level 5 version), or (C) Scorching Ray (level 5 version).

Rend. Melee Attack Roll: +18, reach 15 ft. Hit: 19 (2d8 + 10) Slashing damage plus 14 (4d6) Fire damage.

Firestorm Breath (Recharge 5–6). Dexterity Saving Throw: DC 26, each creature in a 90-foot Cone. Failure: 105 (30d6) Fire damage. If the target is a creature, it is pushed up to 30 feet straight away from the dragon and has the Prone condition. Success: Half damage only.

Call Dragons (5/Day). The dragon causes the Orb of Dragonkind to issue a telepathic call that extends in all directions for 40 miles. Chromatic dragons in range feel compelled to come to the orb as soon as possible by the most direct route. Dragon deities such as Tiamat are unaffected by this call. Chromatic dragons drawn to the orb have a 50 percent chance of being Hostile toward the dragon for compelling them against their will.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 25, +17 to hit with spell attacks):

At Will: Command (level 4 version), Detect Magic, Fear, Invisibility, Mage Hand, Melf's Acid Arrow (level 5 version), Mind Spike (level 5 version), Scorching Ray (level 5 version), Sending, Shatter (level 5 version)
1/Day Each: Cure Wounds (level 9 version), Create Undead, Daylight, Death Ward, Fireball (level 9 version), Geas, Hold Monster (level 7 version), Modify Memory, Scrying, Speak with Dead, Vitriolic Sphere (level 5 version)

Chromatic Burst (5/Day). Dexterity Saving Throw: DC 25, each creature in a 15-foot Emenation originating from the dragon. Failure: 55 (10d10) damage of a type chosen by the dragon: Acid, Cold, Fire, Lightning, or Poison. Success: Half damage.

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.

Cloaked Flight. The dragon uses Spellcasting to cast Invisibility on itself, and it can fly up to half its Fly Speed. The dragon can’t take this action again until the start of its next turn.

Draconic Presence. The dragon uses Spellcasting to cast (A) Command (level 4 version) or (B) Fear. The dragon can’t take this action again until the start of its next turn.

Elemental Rays. The dragon uses Spellcasting to cast (A) Melf's Acid Arrow (level 5 version) or (B) Scorching Ray (level 5 version). The dragon can’t take this action again until the start of its next turn.

Pounce. The dragon moves up to half its Speed or Climb Speed, and it makes one Rend attack. If the attack hits, the target starts burning.