Dexterity +7, Wisdom +10
Perception +17, Stealth +7
+14
Blindsight 60 ft, Darkvision 120 ft
Common, Draconic
Lightning
Traits
Legendary Resistance
4/day (5/day in lair). If the ancient blue dragon fails a saving throw, it can choose to succeed instead.
Actions
Multiattack
The ancient blue dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Shatter (level 3 version).
Rend
Melee Weapon Attack: +16 to hit, reach 15 ft.. Hit: 18 (2d8 + 9) slashing plus 11 (2d10) lightning damage.
Lightning Breath
Recharge 5–6. 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
1 use per day. The ancient blue 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 (level 3 version)
1/Day Each: Scrying, Sending
Legendary Actions
3 legendary actions (4 in its lair). Only one legendary action can be used at a time and only at the end of another creature's turn. This creature regains spent legendary actions at the start of its turn.
Cloaked Flight
The ancient blue 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 ancient blue dragon uses Spellcasting to cast Shatter (level 3 version). The ancient blue dragon can't take this action again until the start of its next turn.
Tail Swipe
The ancient blue dragon makes one Rend attack.
Prepared Spells
Detect Magic, Invisibility, Mage Hand, Shatter, Scrying, Sending, Invisibility, Shatter