Innate Spellcasting. The Rust Lord can cast the following spells without components, using Wisdom as its spellcasting ability (spell save DC 15, +7 to hit with spell attacks):

• at will: heat metal, spiritual weapon (as a manifestation of the spirit of a sentient weapon the rust monster ate)

Ferrous Feast. The rust monster has advantage on attack rolls against any creature wearing armor or a shield made of metal.

Iron Scent. The rust monster can pinpoint, by scent, the exact location of ferrous metal within 30 feet of it.

Legendary Resistance (2/day). If the rust monster fails a saving throw, it can choose to succeed instead.

Rust All Metal. Any weapon made of metal that hits the rust monster corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Ammunition made of metal that hits the rust monster is destroyed immediately after dealing damage.

Rust Regeneration. Once per turn, when the rust monster causes a metal shield, weapon or suit of armor to rust, it regains 10 hit points.

Multiattack. The rust monster makes three attacks: two with its pincers and one with its corrosive bite. It can use its swallow in place of its bite.

Corrosive Bite. Melee Weapon Attack: +10 to hit, one target, reach 5 ft. Hit: 13 (2d6+6) piercing damage plus 9 (2d8) acid damage.
If the target wears either metal armor or carries a metal shield, 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.

Pincer. Melee Weapon Attack: +10 to hit, one target, reach 5 ft. Hit: 17 (2d10+6) piercing damage. The target is grappled (escape DC 15). While grappled, it also is restrained. The rust monster has two pincers, each of which can grapple one creature.

Swallow. The rust monster makes a bite attack against a target it is grappling and that wears armor made of metal. If the attack hits, the target must make a DC 17 Strength saving throw or is swallowed and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the rust monster and it takes 18 (4d8) acid damage at the start of each of the rust monster's turns.

In addition, at the start of each of the rust monster's turns, the creature's metal armor and if it carries a shield made of metal, its shield as well as any metal weapon currently carried by the creature each take a cumulative -1 penalty to the AC they offer or to their damage rolls respectively.  Armor reduced to an AC of 10, a shield that drops to a +0 bonus as well as weapons with a penalty of -5 are destroyed.

If the rust monster takes 30 damage or more on a single turn from a creature inside it, the rust monster must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the rust monster. The rust monster also regurgitates creatures without any metal armor, shields or weapons on them at the end of its turn.

If the rust monster dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Rust Breath (Recharge 5–6). The rust monster breathes acid in a 30-feet cone. Each creature in the area must make a DC 16 Dexterity saving throw, taking 36 (8d8) acid damage on a failure or half as much on a success.

In addition, on a failed save, metal armor worn by the creature and if it carries a shield made of metal, its shield as well as any metal weapon currently carried by the creature each take a cumulative -1 penalty to the AC they offer or to their damage rolls respectively.  Armor reduced to an AC of 10, a shield that drops to a +0 bonus as well as weapons with a penalty of -5 are destroyed.

The rust monster can take 3 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature's turn. The rust monster regains spent legendary actions at the start of its turn.

Pincer. The rust monster makes an attack with its pincers.

No Metal, No Business. The rust monster targets a creature it can see within 60 feet of it that does not wear metal armor and does not carry a shield made of metal. The target must make a DC 15 Wisdom saving throw or be charmed by the rust monster. While charmed in this way, it must use its movement and take the Dash on its turn to move as far away from the Rust Monster as possible. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to this ability for the next 24 hours.

Mandibles (Costs 2 Actions). The rust monster makes a bite attack or uses its swallow.

Command Underlings (Costs 2 Actions). The rust monster commands up to three rust monsters to use their reactions to attack.