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: +14 to hit, reach 15 ft., one target. Hit: (2d10 + 7) piercing damage plus (2d10) lightning damage.   Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: (2d6 + 7) slashing damage.   Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: (2d8 + 7) 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 19 (8 + Prof + Cha) 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.   Storm Spit (Recharge 6). The dragon creates a storm cloud centered somewhere it can see within 60ft of it. The cloud has a 40-foot radius, this cloud heavily obscures as well as deafening any creature while they are within it. The cloud lasts for 1 minute or until the dragon uses this ability again.   Lightning Breath (Recharge 5-6). The dragon exhales lightning in a 120-foot line that is 10 ft. wide. Each creature in that line must make a DC 22 (8 + Prof + Dex) Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.   Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice). In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Reactions

Retributive Arc. If the dragon is hit by an attack it can use it's reaction to send a bolt of lightning at the attacker, who has to make a DC 19 (8 + Prof + Cha) Dexterity saving throw, taking 17 (3d10) lightning damage on a failed save, or half as much damage on a successful one.

Legendary Actions

The 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 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.   Invoke Lightning (Costs 2 Actions). The dragon causes lightning to run through a cloud created by it's Storm Spit and any other storm cloud within 120ft of that cloud, every creature within any of the affected cloud must make a DC 16 (8 + Prof + Wis) Dexterity saving throw taking a number of d10 of lightning damage equal to half the number of effected creatures (rounded up) on a failed save, or half as much damage on a successful one.   Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 ft. of the dragon must succeed on a DC 22 (8 + Prof + Sr) Dexterity saving throw or take 15 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Lair Actions

Lair Actions On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects; the dragon can’t use the same effect two rounds in a row:  

• Part of the ceiling collapses above one creature that the dragon can see within 120 feet of it. The creature must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) bludgeoning damage and be knocked prone and buried. The buried target is restrained and unable to breathe or stand up. A creature can take an action to make a DC 10 Strength check, ending the buried state on a success.
• A cloud of sand swirls about in a 20-foot-radius sphere centered on a point the dragon can see within 120 feet of it. The cloud spreads around corners. Each creature in the cloud must succeed on a DC 15 Constitution saving throw or be blinded 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.
• Lightning arcs, forming a 5-foot-wide line between two of the lair’s solid surfaces that the dragon can see. They must be within 120 feet of the dragon and 120 feet of each other. Each creature in that line must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) lightning damage.

Blue dragons make their lairs in barren places, using their lightning breath and their burrowing ability to carve out crystallized caverns and tunnels beneath the sands. Thunderstorms rage around a legendary blue dragon’s lair, and narrow tubes lined with glassy sand ventilate the lair, all the while avoiding the deadly sinkholes that are the dragon’s first line of defense.   A blue dragon will collapse the caverns that make up its lair if that lair is invaded. The dragon then burrows out, leaving its attackers to be crushed and suffocated. When it returns later, it collects its possessions — along with the wealth of the dead intruders.

Regional Effects

Regional Effects The region containing a legendary blue dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:  

• Thunderstorms rage within 6 miles of the lair.
• Dust devils scour the land within 6 miles of the lair. A dust devil has the statistics of an air elemental, but it can’t fly, has a speed of 50 feet, and has an Intelligence and Charisma of 1 (−5).
• Hidden sinkholes form in and around the dragon’s lair. A sinkhole can be spotted from a safe distance with a successful DC 20 Wisdom (Perception) check. Otherwise, the first creature to step on the thin crust covering the sinkhole must succeed on a DC 15 Dexterity saving throw or fall 1d6× 10 feet into the sinkhole.