Legendary Resistance (3/Day).

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

Tunneler.

The dragon can burrow through solid rock at half its burrowing speed and can leave a 20-foot-diameter tunnel in its wake.

Warp Perception (1/Day).

The dragon can cast mirage arcane, requiring no spell components and using Intelligence as the spellcasting ability.

Actions

Multiattack.

The dragon makes one Bite attack and two Claw attacks.

Bite.

Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 10 (3d6) psychic damage.

Claw.

Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 14 (2d6 + 7) slashing damage.

Disorienting Breath (Recharge 5–6).

The dragon exhales a wave of psychic dissonance in a 90-foot cone. Each creature in that area must make a DC 22 Intelligence saving throw. On a failed save, the creature takes 56 (16d6) psychic damage, and until the end of its next turn, when the creature makes an attack roll or an ability check, it must roll a d8 and reduce the total by the number rolled. On a successful save, the creature takes half as much damage with no additional effects.

Spellcasting (Psionics).

The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 20):

At will: mage hand (the hand is invisible), minor illusion
1/day each: detect thoughts, dispel magic, etherealness, major image, mislead

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.
Emerald Embers (Costs 3 Actions). The dragon creates a dancing mote of green flame around a creature it can see within 60 feet of it. The target must succeed on a DC 20 Dexterity saving throw or take 42 (12d6) fire 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:

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.
Distort Perceptions. The dragon attempts to alter the perceptions of one creature it can see within its lair. That creature must succeed on a DC 15 Intelligence saving throw or take 22 (4d10) psychic damage and have disadvantage on saving throws until the start of its next turn.
Vanish. The dragon becomes invisible until initiative count 20 on the next round.

Regional Effects

The region surrounding a legendary emerald 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 emerald gemstones.
Fiery Sight. Fire and lava within 6 miles of the lair become conduits for the dragon's psionic presence. As an action, the dragon can cast the clairvoyance spell, requiring no spell components and targeting any area of fire or lava in that region.
Subtle Obstruction. Rocks within 6 miles of the dragon's lair sometimes move of their own accord, usually when no one is watching. Often the rocks obstruct the approach to the emerald dragon's lair, with boulders moving to block narrow defiles, way-markers tumbling off the path, or smaller stones shifting beneath travelers' feet to send them tumbling down slopes or into rivers.
Thriving Wildlife. Giant lizards 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 lizards near the lair returns to normal levels over the course of 1d10 days. Rocks immediately stop moving of their own accord. The existing abundance of crystals and emeralds remains, but new ones form at a normal rate.

Ancient Emerald Dragon