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

Siege Monster. The dragon deals double damage to objects and structures.

Nebulous Thoughts. Magical attempts to read the dragon's mind or glean its thoughts fail automatically. 

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) cold damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Cold Breath (Recharge 8–9). The dragon exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 16 Constitution saving throw, taking 36 (8d8) cold damage on a failed save, or half as much damage on a successful one.

Photonic Breath (Recharge 8-9). The dragon exhales a flashing mote of radiant energy that travels to a point the dragon can see within 180 feet of itself, then blossoms into a 30-foot-radius sphere centered at that point. Each creature in the sphere must make a DC 17 Constitution saving throw, taking 44 (8d10) radiant damage on a failed save, or half as much on a successful one.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option 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 a tail attack.

Blinding Radiance (Costs 2 Actions). The dragon emits magical light in a 20-foot-radius sphere centered on itself. Each creature in this area must succeed on a DC 19 Wisdom saving throw or be blinded until the end of its next turn.