Ironclad

Once, he was the last of his tribe, doomed to die in the dreaded spire. However, when offered a demonic pact, he agreed, and a life of servitude, blood, and steel began. Now, his techniques have spread and powers understood. Becoming an Ironclad does not necessarily require one to take a demonic pact, as likely as it may be in this case, but can be due to any number of causes, including containing powers from birth or rigorous, specific training.

Boiling Blood

When you select this subclass at 3rd level, your propensity to recover from wounds has elevated. After rolling initiative, the next time you start your time after having not suffered any damage for one minute, you regain hit points equal to your proficiency bonus.

To do this, you must have suffered damage equal to or greater than your proficiency bonus between rolling initiative and spending one minute without taking damage.

Offering

Beginning at 7th level, you gain the ability to sacrifice a portion of your vitality in order to energize yourself. As a bonus action, you may take damage and lower your Hit Point maximum by an amount equal to your Fighter level, halved. After doing this, the next time you take the Attack action on your turn, you may make one Additional attack. 

When you use your Boiling Blood feature, you may choose to restore any missing Maximum Hit Points instead of healing health. 

Iron Wave

Upon reaching 10th level, you have become practiced with flowing from offense to a sustainable defense. When you deal damage to a creature with a weapon attack on your turn, you may gain temporary Hit Points equal to your Strength modifier. These temporary Hit Points expire at the start of your next turn. 

Blood for Blood

Starting at 15th level, your penchant for battle peaks with retribution. Once per round, when you deal damage to a creature who has dealt damage to you since the end of your last turn, you may cause them to suffer an infernal wound, which lasts until the end of their next turn. 

Creatures with an infernal wound take 1d6 necrotic damage at the start of their turn, and may only use half of their proficiency bonus when rolling ability checks and saving throws with which they are proficient with. 

Limit Break

At 18th level, your internal makeup has evolved and been refined to aid you in times of need. While you are below half of your maximum hit points, your Strength score and maximum for that score is increased by 2, and any healing to your Hit Points you receive gains a bonus equal to your Constitution modifier.