An enormous bipedal creature, reaching more than 8 feet when standing erect. Its body is covered with tan and dull orange chitin plates, and the enlarged head of a rust monster sits atop its broad shoulders. Its antennae have fused with its arms, creating claws which corrode metal with every strike. Powerful legs support its bulky lower abdomen, which is bloated with the eggs of new rust monsters, just waiting to be born live from its grotesque womb.
Tunneler: the Oxide Queen can burrow through solid rock at half its burrowing speed, and leaves a 5-foot-wide, 8-foot-high tunnel in its wake.
Multiattack: the Oxide Queen makes three attacks: one with her claws and one with her bite.
Bite: Melee weapon attack, +8 to hit, reach 5 ft, one target. Hit: 14 (2d8+5) slashing damage.
Destructive Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 8 (2d8+5) slashing damage, and if the target is a creature wearing armor, carrying a shield, or in possession of a magic item that improves its AC, it must make a DC 16 Dexterity saving throw. On a failed save, one such item worn or carried by the creature (the target's choice) magically deteriorates, taking a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield or magic item that drops to a 0 AC increase is destroyed.
Spawn (recharge 6): the Oxide Queen spawns 1 or two rust monsters (flip a coin) into an unoccupied space behind her.