Ancient Blue Dragon

Gargantuan dragon · CR 23

Armor Class 22

Hit Points 481 (26d20+208)

Speed burrow 40 ft., fly 80 ft., 40 ft.

STR 29 +9

DEX 10 +0

CON 27 +8

INT 18 +4

WIS 17 +3

CHA 21 +5

Saving Throws CHA+12, CON+15, DEX+7, WIS+10

Skills perception +17, stealth +7

Damage Immunities lightning

Senses blindsight60 ft., darkvision120 ft. , passive Perception 27

Languages Common, Draconic

Traits

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

Actions

Multiattack.: The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
Bite. Melee/Ranged Weapon Attack: +16 to hit.: Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 11 (2d10) lightning damage.
Claw. Melee/Ranged Weapon Attack: +16 to hit.: Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.
Tail. Melee/Ranged Weapon Attack: +16 to hit.: Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage.
Frightful Presence.: Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 20 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.
Lightning Breath (Recharge 5-6). Melee/Ranged Weapon Attack: +0 to hit.: The dragon exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Legendary Actions

The Ancient Blue Dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The creature regains spent legendary actions at the start of its turn.

Detect.: The dragon makes a Wisdom (Perception) check.
Tail Attack.: The dragon makes a tail attack.
Wing Attack (Costs 2 Actions) (Costs 2 Actions).: The dragon beats its wings. Each creature within 15 ft. of the dragon must succeed on a DC 24 Dexterity saving throw or take 16 (2d6 + 9) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.