Adult Iron Dragon

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Metal Sense. The dragon can sense any and all metals, and whether or not they are magical, within 360 feet of itself.

Multiattack. The dragon can use its Frightful Presence. It then makes three Rend attacks. It can replace one attack with a use of Sleep Breath (if available), Shocking Breath (if available), or Spellcasting to cast Shocking Ray at 2nd level.

Frightful Presence.  Each creature of the dragon's choice within 120 feet that can hear or see it.  DC 18 Wisdom Saving Throw.  Failure: The target is Frightened of the dragon 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 success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.  

Rend. Melee Attack Roll: +12 to hit, reach 10 ft. Hit: 15 (2d8 + 6) Slashing damage plus 7 (1d6+4) Lightning damage.

Shocking Breath (Recharge 5-6, end of own turn). Dexterity Saving Throw: DC 18, each creature in a 60-foot-long Cone. Failure: 45 (10d8) Lightning damage. Success: Half damage.

Sleep Breath (Recharge 5-6, end of own turn). Constitution Saving Throw: DC 18, each creature in a 60-foot Cone. First Failure: The target has the Incapacitated condition until the end of its next turn, at which point it repeats the save. Second Failure: The target has the Unconscious condition for 10 minutes. This effect ends for the target if it takes damage or a creature within 5 feet of it takes an action to wake it.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 18):

At Will: Heat Metal, Stone Shape, Flesh to Iron (Flesh to Stone), Transmute Rock, Wall of Iron, Shocking Ray (Lightning damage Scorching Ray, 4th Level).

Once per Day: Lightning Bolt (5th Level)

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Surge of Power. The dragon uses Spellcasting to cast Shocking Ray at 2nd Level.

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.

Gather Might. The dragon attempts to recharge any of its abilities that did not recharge at the end of its turn.

Instill Terror. The dragon casts Cause Fear at 1st level, using Charisma as its spellcasting modifier (DC18) and requiring no concentration.  Up to 2 targets can be effected by this spell in this way at a time.