Ancient Rust Dragon

Metallic Scent. The rust dragon can pinpoint, by scent, the location of any metal within 120 feet of its current surrounding.

Rust Metal: Any nonmagical weapon made of metal that hits the Rust Dragon 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. Non-magical Ammunition made of metal that hits the Rust Dragon is destroyed after dealing damage. 

Corrosive Acid. After every breath attack, the Rust Dragon Reduces the target's armor bonus AC increase by -1 until the armor does not provide bonus AC anymore. Once the armor does not provide AC anymore the target must succeed a DC 16 CON save or the armor melts off and is dissolved.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Subterranean Dweller. The Rust Dragons are highly resistant to pressure and heat due to their subterranean lairs they are also slightly lava resistant but can only be submerged in lava for 2 minutes before beginning to take (2d8) fire damage per turn while in the lava.

Light Sensitive. Due to living primarily underground most Rust dragons are sensitive to high levels of light so, when the Rust dragon is first exposed to sunlight they take 20 Radiant damage but after being exposed once they do not take damage again until they return underground and then are re-exposed again after 10 days of being underground.

Multiattack. The Rust dragon can use its Frightful Presence then makes three attacks: one with its bite, two with its claws, and one with its tail.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the Rust dragon and aware of it must succeed on a DC 17 Constitution saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a successful save. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Rust dragon's Frightful Presence for the next 24 hours.

Bite. Melee Weapon Attack: +20 to hit, reach 15 ft., one target. Hit: 19 (3d10 + 1) piercing damage plus 10 (2d8) Acid damage.

Claw. Melee Weapon Attack: +20 to hit, reach 10 ft., one target. Hit: 17 (3d10+4) slashing damage.

Tail. Melee Weapon Attack: +20 to hit, reach 20 ft., one target. Hit: 18 (3d8 + 2) bludgeoning damage and also deals 12 (2d8+1) Piercing Damage.

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Corrosive Breath. The Rust Dragon Exhales corrosive acid in a 45-foot cone that is 20 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 82 (14d8) acid damage on a failed save and if the target is reduced to 0 hp on a failed save they are dissolved in acid and become reduced to a puddle of acid, on a successful save the breath attack deals half as much damage.

Acidic Ball.  The Rust Dragon Spits a sphere of acid that is 15 feet tall and 15 feet wide that slowly deforms into an oval that is 17 feet tall and 13 feet wide when it reaches 120 feet where the oval loses its shape and becomes a line shape that is 20 feet long and 10 feet wide. Each creature hit must make a DC 19 Dexterity saving throw, taking 92 (15d8) acid damage on a failed save and if the target is reduced to 0 hp on a failed save they are dissolved in acid and become reduced to a puddle of acid, on a successful save the breath attack deals half as much damage.

Draconic Assault (Recharge 5-6)

Draconic Assault. The Rust Dragon burrows underground and then rushes toward a target area then leaps out of the ground in a radius of 35 feet. Targets at the center of the (1-10 feet) radius are forced to take a DC 20 Dex Save which if fails deals 60 (4d20+10) Bludgeoning damage and the target is knocked up into the air then they are stunned for (1d4) turns when they land. Targets at the edge of the area (11-35 feet) are knocked prone for (1d4) turns and take the same damage and are forced to take the same save. On a successful save the target at the center of the radius take half damage and are only stunned for 1 turn and Targets at the edge of the radius take 1/4 of the damage and are only knocked prone for 1 turn. The rust dragon can then attempt to bite one of the stunned targets as a bonus action.

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

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The Rust dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 21 Dexterity saving throw or take 15 (2d8 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.