Legendary Resistance (3/Day).

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

Actions

Multiattack.

The dragon makes one Bite attack and two Claw attacks.

Bite.

Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 11 (2d10) radiant damage.

Claw.

Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail.

Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 10 (1d8 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 21 Strength saving throw or be knocked prone.

Breath Weapon (Recharge 5–6).

The dragon uses one of the following breath weapons:

Dream Breath. The dragon exhales mist in a 90-foot cone. Each creature in that area must succeed on a DC 21 Constitution saving throw or fall unconscious for 10 minutes. This effect ends for a creature if the creature takes damage or someone uses an action to wake it.
Moonlight Breath. The dragon exhales a beam of moonlight in a 120-foot line that is 10 feet wide. Each creature in that area must make a DC 21 Dexterity saving throw, taking 60 (11d10) radiant damage on a failed save, or half as much damage on a successful one.

Spellcasting.

The dragon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 23):

At will: faerie fire
2/day each: calm emotions, dispel magic, invisibility, revivify

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.

Tail. The dragon makes one Tail attack.
Cast a Spell (Costs 2 Actions). The dragon uses Spellcasting.

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:

Banish into Dream. The dragon targets a creature it can see within 120 feet of it and attempts to send that creature to a dream plane. The target must succeed on a DC 15 Charisma saving throw or be banished to a harmless demiplane until initiative count 20 on the next round. While there, the target is stunned. When the effect ends, the target reappears in the space it left or in the nearest unoccupied space.
Compulsive Dance. The dragon targets a creature it can see in its lair, and a merry waltz begins to play that only the target can hear. The target must succeed on a DC 15 Intelligence saving throw or dance until initiative count 20 on the following round. The dancing creature is incapacitated and must use all its movement to dance.
Disorienting Visions. Disorienting illusory images flit through the dragon's lair. Each creature in the lair must succeed on a DC 20 Wisdom saving throw or have disadvantage on ability checks until initiative count 20 on the following round.

Regional Effects

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

Dream Communication. Whenever a creature that can understand a language sleeps or enters a state of trance or reverie within 6 miles of the dragon's lair, the dragon can establish telepathic contact with that creature and converse with it in its dreams. The creature remembers its conversation with the dragon upon waking.
Planar Transition. The veil between planes is thinned near a moonstone dragon's lair. Various portals link the Material Plane, the Feywild, and the Border Ethereal within 6 miles of the lair. Creatures can pass through a portal in either direction by spending 5 feet of movement.

If the dragon dies, any portals near the lair close immediately.

Ancient Moonstone Dragon