Adult Blue Dragon
Huge Dragon, lawful evil
15,000 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: +12 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 5 (1d10) lightning damage.
Claw
Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) slashing damage.
Tail
Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage.
Frightful Presence
Each creature of the dragon's choice that is within 120 ft. of the dragon and aware of it must succeed on a DC 17 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 90-foot line that is 5 ft. wide. Each creature in that line must make a DC 19 Dexterity saving throw, taking 66 (12d10) 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 10 ft. of the dragon must succeed on a DC 20 Dexterity saving throw or take 14 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Quick Reference
Running This Encounter
An Adult Blue Dragon is a devastating opponent that should be treated as a major boss encounter. Open with a lightning breath attack to damage the entire party, then use legendary actions to maintain pressure between turns. Leverage the dragon's high AC and massive HP pool by positioning it where melee attackers must approach carefully, and use lair actions if this is its domain. Force the party to make meaningful tactical decisions about action economy and resource management.
Combat Tactics
The Adult Blue Dragon prioritizes dealing area damage with its lightning breath, recharging it when possible to soften the party's defenses. It uses its flight and high mobility to stay out of melee range while using legendary actions like Wing Attack to control battlefield positioning and knock enemies prone.
Environment & Setting
An ideal lair features high ceilings for aerial combat, conductive terrain like metal or water that amplifies lightning effects, and multiple perches or pillars for tactical positioning. Consider incorporating storm clouds, electrical phenomena, or a mountain peak to enhance the dragon's natural affinity.