Ancient Blue Dragon
Gargantuan Dragon, lawful evil
50,000 XP
Special Abilities
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 Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 11 (2d10) lightning damage.
Claw
Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.
Tail
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)
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
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.
Quick Reference
Running This Encounter
The Ancient Blue Dragon is a legendary creature that should feel overwhelming in power. Use its lair actions and legendary actions to control the battlefield, taking multiple turns per round. Position it to maximize its breath weapon effectiveness and force the party into difficult tactical decisions. Leverage its intelligence and cunning to employ hit-and-run tactics, utilizing flight and ranged lightning attacks.
Combat Tactics
The dragon opens with its lightning breath to damage multiple targets and separate the party. It uses its flying speed to maintain distance while attacking with its Multiattack, prioritizing spellcasters and damage dealers. When damaged significantly, it uses legendary actions to dodge, cast spells, and make tail attacks to control positioning.
Environment & Setting
Position this encounter in an open-air outdoor setting like a mountaintop aerie, stormy plain, or ancient temple courtyard where the dragon can utilize its full flying capabilities. Incorporate conductive terrain or structures that amplify lightning damage to create a thematic advantage.