Slayer

Blood is the currency of the battlefield, and the reason why you fight. Your style edges on brutality, focusing only on the essence of victory and nothing more; to be a Slayer is to stain your hands eternally red for the thrill. Fighters of this archetype usually abuse their disciplined training for this purpose, hungrily seeking battle for more worthy combatants to test their prowess.

Credits for Subclass: Novronian on Reddit

Killer's Intent

At 3rd level, you strike with killing intent. Once per turn, when you take the Attack action and make an attack with a melee weapon, you can choose to attack with disadvantage. If you do, regardless of whether or not you hit, the target must make a Wisdom saving throw (DC equal to 8 + proficiency bonus + your Constitution modifier). On a failed save, the creature is frightened until the start of your next turn.

You can use this feature a number of times equal to your Constitution modifier (minimum of once), regaining expended uses on a short or long rest.

Killer's Instinct

Also at 3rd level, you'll always reach the next victim. Immediately before you make a melee weapon attack, you can use your bonus action to move up to your speed towards the target. If you do and that attack hits a frightened creature, you gain temporary hit points equal to half your fighter level (rounded up) + your Constitution modifier, which lasts until the start of your next turn.

Blood Hound

At 7th level, your indulgence in the battlefield has steeled your will and sharpened your senses. You can't be surprised, and creatures within 60 feet of you, if they have blood, can't benefit from being hidden or invisible against you if they are below half their hit point maximum.

Thrill of Battle

At 10th level, you thrive in the thrill of battle. You have advantage on initiative checks. Also, there isn't a moment when you aren't fighting. When you use your Second Wind, you can also make a single melee attack with advantage against a creature within reach. On a hit, the target can't take reactions until the start of your next turn.

Harbringer

At 15th level, the sight of you signifies a slaughter. When you use your Killer's Instinct to move towards a target, your movement doesn't provoke opportunity attacks and ignores difficult terrain. Also, a creature can't gain advantage on saving throws against being frightened by you.

Last Breath

At 18th level, you've breathed your last breath as a person, the hunt for battle is all that drives you forward. After succeeding on a death saving throw, you can choose to stand up and act as if you had started your turn conscious. You remain conscious until the end of your turn. The first attack you make during this turn is made with advantage and deals an additional 5d10 damage of the weapon's type on a hit. While you have 0 hit points during this turn, taking damage causes death saving throw failures as normal, though three death saving throw failures doesn't kill you. At the end of your turn, you fall unconscious if you still have 0 hit points or die if you have three death saving throw failures.

Once you use this feature, you can't use it again until you complete a long rest.