Tunneler. The dragon can burrow through solid rock at half its burrowing speed and leaves a 5-foot-diameter tunnel in its wake.

Unusual Nature. The dragon doesn't require air.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit:5 (1d6 + 2) piercing damage plus 3 (1d6) cold damage.

Cold Breath (Recharge 5–6). The dragon exhales a blast of frost in a 15-foot cone. Each creature in the cone must make a DC 13 Constitution saving throw. On a failed save, the creature takes 13 (3d8) cold damage, and its speed is halved until the end of its next turn. On a successful save, the creature takes half as much damage, and its speed isn't reduced.

Phase (2/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.