Adult Crystal Dragon

Huge dragon (gem), chaotic neutral

Armor Class 16 (natural armor)

Hit Points 172 (15d12 + 75)

Speed 40ft Fly: 80ft Burrow: 40ft Climb: 40ft

STR

21 +5

DEX

12 +1

CON

21 +5

INT

18 +4

WIS

15 +2

CHA

19 +4

Saving Throws Dex +5, Con +9, Wis +6, Cha +8

Skills Perception +10, Stealth +9, Survival +6

Damage Resistances cold, radiant

Senses Blindsight 60 ft., Darkvision 120 ft., passive Perception 20

Languages Common, Draconic, telepathy 120 ft.

Challenge 12 (8,400 XP)

(Psionics). The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 16):

At will: Dancing Lights, Guidance

1/day: Command, Divination, Hypnotic Pattern, Lesser Restoration

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: +9 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage plus 4 (1d8) radiant damage. Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) slashing damage. Scintillating Breath (Recharge 5–6). The dragon exhales a burst of brilliant radiance in a 60-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 40 (9d8) radiant damage on a failed save, or half as much damage on a successful one. The dragon then gains 15 temporary hit points by absorbing a portion of the radiant energy.

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. Starlight Strike (Costs 3 Actions). The dragon releases a searing beam of starlight at a creature that it can see within 60 feet of it. The target must succeed on a DC 17 Dexterity saving throw or take 31 (9d6) radiant 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.
Ice Passage. The dragon can open a passage through a wall of ice or snow that is up to 5 feet thick, creating an opening up to 30 feet wide and high.
Starlight's Gleam. The dragon chooses a point it can see in the lair. Gleaming starlight radiates from that point to fill a 10-foot-radius sphere with dim light. Each creature other than the dragon in that area when the light appears must succeed on a DC 15 Dexterity saving throw or take 13 (2d12) radiant damage and be outlined in the glow. Attack rolls made against an outlined creature have advantage, and the creature can't hide or benefit from being Invisible. The starlight and the glow around any creature fades on initiative count 20 on the next round.

Regional Effects

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

Clear Skies. The skies above a crystal dragon's lair remain clear and free of precipitation unless magically altered. Winds blow lightly, posing little threat to those approaching the lair, and visibility is the best possible for the time of day.
Crystal Profusion. Plentiful quartz crystals form in natural stone within 6 miles of the lair, particularly in places where natural light can shine on the crystals.
Icy Sight. Ice and quartz 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 ice or quartz crystals in that region.
Positive Energy. Any creature that finishes a long rest within 6 miles of the lair regains two additional spent Hit Dice.
Thriving Wildlife. Animal populations flourish within 6 miles of the lair. Ability checks made to forage for food by hunting, fishing, or trapping in that area are made with advantage.

If the dragon dies, the animal population near the lair returns to normal levels over the course of 1d10 days. The increased Hit Die recovery ends immediately. The existing abundance of quartz crystals remains, but new crystals form at a normal rate.