Adult Zirak

Rock And Metal Devourer. Born from a 10 cm diameter egg, they grows and become 15 ft. longer. They can now eat entire rocks, digging into mountains and terrains.

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

Corrosive Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 9 (2d6 + 2) piercing damage plus 5 (2d4) acid damage. The Adult Zirak has a corrosive mucus in its mouth and it can digest rocks or metal plates: a 1 foot cube of any nonmagical object, that is not carried or worn, is consumed and swallowed when hit this way. If the object is being worn or carried, it takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed. 

Excavate (Recharge 5/6). As an action, an Adult Zirak can burrow through solid rock, finishing all of its acid in this action. It moves up to a maximum given by half of the burrow speed and then emerges from the ground, trying to eat its prey. Any creature placed where the Adult Zirak emerges must succeed a DC 15 (Dex) Saving throw and take 25 (6d6 + 4) piercing damage on a failure, half as much on a success.