Gargantuan dragon, lawful evil
- Armor Class 22 (Natural Armor)
- Hit Points 481 (26d20+208)
- Speed 40 ft., burrow 40 ft., fly 80 ft.
- Saving Throws Dex +7, Con +15, Wis +10, Cha +12
- Skills Perception +17, Stealth +7
- Damage Immunities Lightning
- Senses Blindsight 60 Ft., Darkvision 120 Ft., passive Perception 27
- Languages Common, Draconic
- Challenge 23 (50,000 XP)
- Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
- Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
- Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: (2d10 + 9) piercing damage plus (2d10)lightning damage.
- Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: (2d6 + 9) slashing damage.
- Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: (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). 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.
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. Ancient Blue Dragon regains spent legendary actions at the start of their turn.
- Detect.The dragon makes a Wisdom (Perception) check.
- Tail Attack.The dragon makes a tail attack.
- Wing Attack (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.