Siege Monster.

The auger deals double damage to objects and structures.

Tunneler.

The auger can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Actions

Flaming Drill.

Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 7 (2d6) fire damage. If the auger moves at least 20 feet in a straight line toward the target immediately before the hit, the target takes an additional 11 (2d10) piercing damage, and if the target is a creature, it must succeed on a DC 17 Strength saving throw or have the prone condition.

Bonus Actions

Burst of Heat (Recharge 5–6).

The auger releases an intense burst of heat in a 30-foot-radius sphere centered on itself. This heat spreads around corners. Each creature in this area must make a DC 17 Constitution saving throw, taking 13 (3d8) fire damage on a failed save, or half as much damage on a successful one.

Fiendish Auger