Ancient Blue Dragon

Gargantuan Dragon, lawful evil

AC: 22 (natural armor)
HP: 481 (26d20 + 208)
Speed: 40 ft., fly 80 ft., burrow 40 ft.

STR	DEX	CON	INT	WIS	CHA
29 (+9)	10 (+0)	27 (+8)	18 (+4)	17 (+3)	25 (+7)

Saves: Str +9, Dex +7, Con +8, Int +4, Wis +10, Cha +7
Skills: Perception +17, Stealth +7
Immunities: lightning
Senses: blindsight 60 ft., darkvision 120 ft., passive Perception 27
Languages: Common, Draconic
CR: 23 (XP 50,000)

Traits

Legendary Resistance (4/Day, or 5/Day in Lair).

If the dragon fails a saving throw, it can choose to succeed instead.

Actions

Multiattack.

The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Shatter (level 3 version).

Rend.

Melee Attack Roll: +16, reach 15 ft. 18 (2d8 + 9) Slashing damage plus 11 (2d10) Lightning damage.

Lightning Breath.

Dexterity Saving Throw: DC 23, each creature in a 120-foot-long, 10-foot-wide Line. Failure: 88 (16d10) Lightning 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 22):

At Will: Detect Magic, Invisibility, Mage Hand, Shatter
1/Day Each: Scrying, Sending

Legendary Actions

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.

Sonic Boom.

The dragon uses Spellcasting to cast Shatter (level 3 version). The dragon can't take this action again until the start of its next turn.

Tail Swipe.

The dragon makes one Rend attack.