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 leaves a 20-foot-diameter tunnel in its wake.

Unusual Nature.

The dragon doesn't require air.

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: 15 (2d6 + 8) piercing damage plus 7 (2d6) cold damage.

Claw.

Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail.

Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage.

Cold Breath (Recharge 5–6).

The dragon exhales a blast of frost in a 90-foot cone. Each creature in the cone must make a DC 21 Constitution saving throw. On a failed save, the creature takes 36 (8d8) cold damage, and its speed is reduced to 0 until the end of its next turn. On a successful save, the creature takes half as much damage, and its speed isn't reduced.

Bonus Actions

Phase (3/Day).

The dragon becomes partially incorporeal for as long as it maintains concentration on the effect (as if concentrating on a spell). While partially incorporeal, the dragon has resistance to bludgeoning, piercing, and slashing damage.

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 Attack. The dragon makes one Tail attack.
Treacherous Ice. Magical ice covers the ground in a 20-foot radius centered on a point the dragon can see within 120 feet of itself. The ice, which is difficult terrain for all creatures except lunar dragons, lasts for 10 minutes or until the dragon uses this legendary action again.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 21 Dexterity saving throw or take 12 (1d8 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its speed without provoking opportunity attacks.

Regional Effects

The region containing an ancient or adult lunar dragon's lair is warped by the dragon's magic, which produces one or more of the following effects:

Black Frost. A thin layer of black frost covers the ground, which kills all ordinary plants growing within 6 miles of the lair if the dragon is ancient, or within 3 miles if the dragon is an adult.
Haunting Moan. A haunting sound gets louder or fainter (dragon's choice) the closer one gets to the lair. The moan is audible 6 miles from the lair if the dragon is ancient, or 3 miles if the dragon is an adult.
Moon Devils. Swirling funnels of dust and frost known as moon devils crisscross the area within 1 mile of the lair, intercepting other creatures they encounter. A moon devil is a free-willed air elemental that deals cold damage instead of bludgeoning damage.

If the dragon dies, the moaning stops, the moon devils dissipate, and the black frost disappears over the course of 1d10 days, allowing plant life in the area to recover.

Ancient Lunar Dragon