Legendary Resistance (3/Day).

If the dragon fails a saving throw, it can choose to succeed instead.

Spider Climb.

The dragon can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Tunneler.

The dragon can burrow through solid rock at half its burrowing speed and can leave a 10-foot-diameter tunnel in its wake.

Actions

Multiattack.

The dragon makes one Bite attack and two Claw attacks.

Bite.

Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 5 (1d10) thunder damage.

Claw.

Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) slashing damage.

Debilitating Breath (Recharge 5–6).

The dragon exhales a pulse of high-pitched, nearly inaudible sound in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw. On a failed save, the creature takes 44 (8d10) thunder damage and is incapacitated until the end of its next turn. On a successful save, the creature takes half as much damage and isn't incapacitated.

Spellcasting (Psionics).

The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 17):

1/day each: dissonant whispers, hold monster, meld into stone, telekinesis

Bonus Actions

Change Shape.

The dragon magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

Psychic Step.

The dragon magically teleports to an unoccupied space it can see within 60 feet of it.

Legendary Actions

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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 its turn.

Claw. The dragon makes one Claw attack.
Psionics (Costs 2 Actions). The dragon uses Psychic Step or Spellcasting.
Telekinetic Fling (Costs 3 Actions). The dragon chooses one Small or smaller object that isn't being worn or carried that it can see within 60 feet of it, and it magically hurls the object at a creature it can see within 60 feet of the object. The target must succeed on a DC 17 Dexterity saving throw or take 31 (9d6) bludgeoning damage.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon can take one of the following lair actions; the dragon can't take the same lair action two rounds in a row:

Awesome Thunder. A thunderous detonation of sound that can be heard up to 300 feet away surrounds one creature in the lair that the dragon can see. That creature must succeed on a DC 15 Constitution saving throw or take 13 (3d8) thunder damage and be stunned until the end of its next turn.
Beguiling Whisper. The dragon telepathically whispers to one creature within range of the dragon's telepathy. The creature must succeed on a DC 15 Wisdom saving throw or be charmed by the dragon until initiative count 20 on the next round. A creature charmed in this way obeys to the best of its ability any command the dragon issues that isn't directly harmful to the creature.
Stone Passage. The dragon touches a section of stone up to 30 feet in any dimension. The dragon can shape the stone to open or close a passage through a wall, as long as the wall is less than 10 feet thick.

Regional Effects

The region surrounding a legendary sapphire dragon's lair is altered by the dragon's magic, creating one or more of the following effects:

Crystal Profusion. Natural stone within 6 miles of the lair grows plentiful crystal formations and veins of sapphire gemstones, particularly underground.
Stony Sight. Natural stone within 6 miles of the lair is a conduit for the dragon's psionic presence. As an action, the dragon can cast the clairvoyance spell, requiring no spell components and targeting any natural stone formation in that region.
Telepathic Enhancement. The dragon's psionic energy enhances the mental powers of other creatures. Any creature capable of telepathic communication has its telepathy range doubled while within 1 mile of the lair. This includes creatures with innate telepathy and magical telepathy such as the Rary's telepathic bond spell.
Thriving Wildlife. Giant spiders (a sapphire dragon's favorite prey) are attracted to the area within 6 miles of the lair and settle there in large numbers.

If the dragon dies, the population of giant spiders in the region returns to normal levels over the course of 1d10 days. The enhancement of telepathic abilities ends immediately. The existing abundance of crystals and sapphires remains, but new ones form at a normal rate.

Adult Sapphire Dragon