Underdog

This subclass is built around gaining a fighting chance when your against the odds. Fighters who undertake this archetype understand that there will always be a bigger and more lethal enemy than themselves, and so centre there training in being able to withstand hardship and exploit the overconfidence of their enemies relentlessly.

Unexpected Comeback

At 3 level, when you miss a weapon attack on a creature, you may use your reaction to cause that creature to become vulnerable to damage from your attacks until the end of your next turn. You may use this ability a number of times equal to your proficiency bonus. You regain all uses on a long rest.

Brutal Determination

At 7th level, you learn to expand your boundaries and maximize your efforts, so that you can get the most out of every chance you get. When you would make a weapon attack, not during your turn, you can apply your extra attack feature.

Tenacity

Starting at 10th level, you spirit bolsters you to press on against the strife you face in battle, allowing you to harden yourself against restraint. When you activate your ­Second wind ability, you may expend up to half of your remaining hit die and gain that many temporary hit points in addition to its additional healing value. Additionally, while you have these temporary hit points you have advantage on ability checks or saving throws to avoid becoming frightened, grappled, restrained, stunned, or knocked prone.

Unwavering Determination

At 15th level, your stamina has built greatly and you can now push yourself even farther than before. You gain an additional reaction. You cannot use two reactions for the same instance. i.e, an opportunity attack uses only 1 reaction, but if an opportunity attack would become available in a separate instance, the second reaction may be used. Additionally, if you hold your action and the trigger doesn’t resolve, you gain a +1 to all of your attack rolls, ability checks and saving throws until the end of your turn.

Even The Playing Field

At 18th level, you understand that your enemy's strikes against you cannot go unchallenged. You gain advantage when attacking a target that has successfully damaged you within the previous round.

Solidarity

At 18th level, your many victories have given you certainty in your worth and have emboldened your allies. When you, or an ally of your choice who has seen you in battle, rolls a natural 1 on an ability check you can choose to convert that into a natural 20 instead. You may only use this ability once per day for each affected creature.