Way of the Metallic Fist is a Monk subclass for DnD5e that allows the monk to turn their skin into metal to temporarily increase defense and striking power. The use of these abilities requires Ki as most monk abilities do so a high wisdom modifier is recommended along with a strength being the second highest for a more powerful unarmed strike. 

Metallic Fists

Spending two Ki points towards this ability allows you to turn your fists metallic. This ability adds 1d4 damage to your unarmed strike and increases to 2d4 at level 9.

Needles

When attacked a portion of your body hardeneds and sprouts spikes back at the attacker with the same force as they attacked you. Spend two Ki points when an enemy lands a successful hit on you they receive equal damage.

Iron Skin

You gain a bonus to your unarmored defense. Add +2 to your ac. 

Adamantine Body

Your entire body hardens into adamantine. You gain resistance to Bludgeoning, slashing, piercing, and fire damage, you are also harder to move as you are significantly heavier (creatures must make a strength check of 18 or higher). Spending 4 Ki points causes any unarmed strike against you is now ineffective and you also gain the bonus of metallic fist (extra d4 damage depending on level). However you are weaker to lightning damage in this state and will take double damage.