Path of the Forged Hide

These Barbarians have learned ways on how to turn their skins from flesh to metallic. They have gained their metallic skin in many ways, such as stepping into pools of liquid alloy. 

Metallic Skin

Starting at 3rd level, whenever you rage your skin turns into a flexible metal. You gain a bonus to your Armor Class equal to half your proficiency modifier (rounded down), and have disadvantage on Dexterity (Stealth) checks. Once your rage ends, your skin becomes normal, and the above benefits end as well.

Alloy

When you choose this subclass at 3rd level, you must choose one type of metal (Alloy) that your skin transforms into. There are three metals to choose from, Adamantine, Mithril and Steel. Each have their own benefits, and once you choose a metal, you continue with that particular metals benefits only.

Adamantine - You gain temporary Hit Points equal to your Barbarian level whenever you start a rage. Additionally, you can use a bonus action to regain these temporary hit points.

Mithril - You no longer have disadvantage on your Dexterity (Stealth) checks, and you may also use the Dash or Disengage action as a bonus action.

Steel - Your unarmed strikes do 1d4 plus Strength modifier in damage while not raging, and you may make an extra unarmed strike as a bonus action. While raging and in your steel form, your unarmed strikes do 1d6 damage plus your Strength modifier.

Tough & Flexible

Starting at 6th level, you continue to gain benefits from your chosen alloy, Adamantine, Mithril or Steel.

Adamantine - Any critical hit scored against you, is considered a normal hit.

Mithril - If an ally that you can see within 30 feet of you is hit by a melee attack, then as a reaction, you may move up to half of your movement speed without provoking attacks of opportunity towards the attacker. If this movement puts you within melee range of the attacker, you may also make one attack against that creature.

Steel - Your unarmed strikes while raging are considered magical for the purposes of overcoming damage resistance. Additionally, your unarmed strikes while raging improve to 1d8 in damage.

Tempered In Fire

Starting at 10th level, as all metal is tempered and forged in fire, so do all metal types gain resistance to fire damage.

Might of Metal

Starting 14th level, you continue to gain benefits from your chosen alloy, Adamantine, Mithril or Steel.

Adamantine - You can use your reaction, when taking damage, to reduce the damage to zero. This ability can be used a number of times equal to your Strength modifier. You regain all uses of this ability    after you finish a long rest.

Mithril - You gain Evasion. When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

Steel - While raging, your unarmed strikes now do 1d12 base damage. Additionally, as a reaction to being hit with a melee attack that is a critical hit, you may strike your attacker back once per round.