Your size is depends on your Ancestry. Small for Nickel, or Medium otherwise.

Your lineage stems from a dragon progenitor. Choose the kind of dragon from the Draconic Ancestors table. Your choice affects your Breath Weapon, Damage Resistance, and Ferrous Heritage traits as well as your appearance.

When you take the Attack action on your turn, you can replace one of your attacks with an exhalation of magical energy in either a 15-foot Cone or a 30-foot Line that is 5 feet wide (choose the shape each time). The damage type and dice depend on your Ancestry.

Chromium

Crystalline Breath. Each creature in that area must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature takes 1d10 Cold damage. On a successful save, a creature takes half as much damage. This damage increases by 1d10 when you reach character levels 5 (2d10), 11 (3d10), and 17 (4d10).

Cobalt

Magnetic Breath. Each creature in that area must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature takes 1d10 Force damage. On a successful save, a creature takes half as much damage. This damage increases by 1d10 when you reach character levels 5 (2d10), 11 (3d10), and 17 (4d10).

Iron

Spark Breath. Each creature in that area must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature takes 1d6 Fire damage and 1d4 Lightning Damage. On a successful save, a creature takes half as much damage. The Fire damage increases by 1d6 when you reach character levels 5 (2d6), 11 (3d6), and 17 (4d6), and the Lightning damage increases by 1d4 when you reach character levels 5 (2d4), 11 (3d4), and 17 (4d4).

Nickel

Acidic Breath. Each creature in that area must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature takes 1d10 Acid damage. On a successful save, a creature takes half as much damage. This damage increases by 1d10 when you reach character levels 5 (2d10), 11 (3d10), and 17 (4d10).

Tungsten

Superheated Sand Breath. Each creature in that area must make a Constitution saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature takes 1d6 Bludgeoning damage and 1d4 Fire Damage. On a successful save, a creature takes half as much damage. The Bludgeoning damage increases by 1d6 when you reach character levels 5 (2d6), 11 (3d6), and 17 (4d6), and the Fire damage increases by 1d4 when you reach character levels 5 (2d4), 11 (3d4), and 17 (4d4).

You can use this Breath Weapon a number of times equal to your Proficiency Bonus, and you regain all expended uses when you finish a Long Rest.

You have damage Resistance determined by your Draconic Ancestry trait.

You have Darkvision with a range of 60 feet.

You gain unique traits depending on your ancestry.

Chromium

Blinding Flash. At character level 5, when you are hit with an attack from a creature within 30 feet, you can magnify the reflective property of your scales as a Reaction. The creature must make a Constitution saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus), or take 2d6 Radiant damage and have the Blinded condition for until the end of its next turn on a fail, or half as much damage only on a success. This damage increases by 1d6 when you reach character levels 11 (3d6) and 17 (4d6).

Once you use this Reaction, you may not do so again until you finish a Long Rest.

Cobalt

Rimedeath Fog. At character level 5, when you take the Attack action on your turn, you can replace one of your attacks with an exhalation of toxic, freezing miasma in a 15 foot cone. Each creature within the cone must make a Constitution saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus), taking 2d6 Poison damage and 2d4 Cold damage and have the Poisoned condition for 1 minute on a fail, or half as much damage only on a success. At the end of its turns, a creature Poisoned in this way can repeat the save, ending the condition on a success. The Poison damage increases by 1d6 when you reach character levels 11 (3d6) and 17 (4d6), and the Cold damage increases by 1d4 when you reach character levels 11 (3d4) and 17 (4d4).

Once you use this feature, you may not do so again until you finish a Long Rest.

Iron

Powerful Build. You have Advantage on any ability check you make to end the Grappled condition. You also count as one size larger when determining your carrying capacity.

Searing Presence. At character level 5, when you take the Attack action on your turn, you can replace one of your attacks with a focused blast of your internal heat at one creature within 30 feet of you. Any metal being worn or carried by the creature starts to glow red-hot. The creature must succeed on a Constitution saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus), taking 2d6 (scales) Fire damage and drop any metal object it's holding on a failed save, or half as much damage only on a success. The damage increases by 1d6 when you reach character levels 11 (3d6) and 17 (4d6).

Once you use this feature, you may not do so again until you finish a Long Rest.

Nickel

Small, Swift, and Sturdy. You have advantage on saving throws you make to avoid or end the Poisoned condition, you have a swimming speed equal to your walking speed, and you can breathe air and water.

Misdirect. At character level 5, when an attack roll misses you, or you pass a Dexterity saving throw, you can use your Reaction to move up to your Speed without provoking opportunity attacks.

You can use this Reaction a number of times equal to your Proficiency Bonus, and you regain all expended uses when you finish a Long Rest.

Tungsten

Immolation. At character level 5, when you take the Attack action on your turn, you can replace one of your attacks with a concentrated blast of fire. One creature within 30 feet of you must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus), taking 2d6 Fire damage on a failed save or half as much damage on a success. On a failed save the target also catches fire, taking 2d4 fire damage at the start of each of their turns for 1 minute, or until someone takes an action to douse the fire. The initial damage increases by 1d6 when you reach character levels 11 (3d6) and 17 (4d6), and the burning damage increases by 1d4 when you reach character levels 11 (3d4) and 17 (4d4).

Once you use this feature, you may not do so again until you finish a Long Rest.