Spartan

Spartans are the most enduring soldiers imaginable. They exist to fight, however they live by a strict code of honor: no soldier is superior to another, a calm mind is stronger than a reckless one; and they never, ever, drop their shield. You will fight to your last breath, but you will not put those around you in unnecessary danger. Others look at you and think of you as a brute, but what they don't see is the brilliant tactician that lives within you. You are not a Barbarian; you are a Fighter and a warrior. You strike with precision without giving the enemy an easy strike against you.

Phalanx

At 3rd level, you have honed your capabilities with a shield. As a bonus action, you can use half your movement speed to raise your shield in front you, granting yourself half cover from attacks until the start of your next turn. Additionally, assuming you have not moved this turn, you may instead use an action to spend all of your movement speed to plant yourself in a guarded stance with your shield, granting yourself and nearby allies within 5 feet of you three-quarters cover until the start of your next turn.

Riposte

Starting at 7th level, you have mastered the art of deflect-and-counterattack. When a creature within 5 feet of you that you can see hits you with a melee attack, you can use your reaction to deflect the incoming attack with your shield. Doing so reduces the incoming damage by 1d12 + the total AC bonus from your shield + your Fighter level.
If doing so reduces the damage to 0, you may immediately make an attack against that creature as part of the same reaction.

Shield Bash

At 10th level, your experience with your shield has taught you new methods of delivering powerful blows. Whenever you take the Attack action on your turn, you may replace one of your attacks with this ability. When you do so, you thrust your shield into the enemy's face, forcing them to make a Constitution saving throw equal to 8 + your proficiency bonus + your Strength modifier. On a failed save, all melee attacks made against that creature have advantage until the end of that creature's next turn.

Victory or Death

Starting at 15th level, you are braver than most; never allowing yourself to retreat in battle. When you fall below half of your maximum hit points, you gain a number of temporary hit points. The total of these hit points equal your Fighter level + your proficiency bonus.
While you have temporary hit points granted to you by this feature, you have advantage on melee attack rolls.

Once you use this trait, you cannot use it again until you finish a long rest, or if you roll a natural 20 on a death saving throw.

Bred for Battle

At 18th level, your expertise in battle has granted you stamina beyond any other warrior. You gain one additional use of your Action Surge. Additionally, while wielding a weapon with the "versatile" property, you may now use the higher damage die of that weapon whilst holding it in one hand. This feature only functions while a shield is equipped.

Action Surge feature cannot be activated more than once per turn