Executioner

Trained in the grim art of delivering death as a means of reprisal to all forms of criminals and miscreants, lumbering men and women of great physical might who hide their faces behind masks. Executioners are feared for the many ways in which they are versed in ending lives, often with a single strike of their blades. They are instruments of death, capable in equal parts of serving as oppressors or bringers of justice, but regardless of which role history  assigns them they are are a stern reminder that no crime goes unpunished.  

(This subclass was made by Reddit user, not myself. However I cannot find their name to give them credit. I only wished to share their creation with others.)

Fear of the Noose

Your presence brings with it the creeping sense of death to those that look upon you. You gain proficiency in the Intimidation skill and if you are already proficient you then gain expertise in that skill.

Path to the Gallows

Starting at 3rd level you can render your enemies helpless, putting them in the optimal position to strike the final blow. When you hit a creature with a weapon attack on your turn, you can force that creature to make a Strength saving throw DC= 8 + STR modifier + your proficiency bonus. On a failure the target is knocked prone and its speed is reduced to 0 until the end of your next turn. You can use this feature a number of times equal to your strength modifier and you regain all uses once you finish a long rest.

Execution

You've learned to strike at the vitals and the taking of ones head. At 3rd level you can deal a devastating blow to your enemies while they are most vulnerable, potentially ending their life. Once on each of your turns, when you hit a creature with a weapon attack while you have advantage and the target's speed is 0, you can choose to roll a D20, if the roll is lower or equal to your fighter level the attack is a critical hit.

Swift Mercy

Beginning at 7th level you can efficiently dispose of multiple targets at once. When you reduce a creature to 0 hit points or when you score a critical hit, you can use your bonus action to move up to half your speed and make one weapon attack against a different target.

Inescapable End

Starting at 10 level very few are capable of avoiding your sentence. Creatures produce attacks of opportunity from you whenever they cast a spell or produce a spell-like effect that would allow them to move or teleport away from your reach. If the attack of opportunity hits the spell or spell-like effect ends and the creature remains in the same place.

Death Dealer

Starting at 15th level when you use your action surge, you can use your execution feature once that turn without needing to fulfill its conditions.

Final Blow

At 18th level, your strikes become a guaranteed death sentence. When you score a critical hit, you can force the target to make a Constitution saving throw, DC= 8+your STR modifier + your proficiency bonus. If the target fails it is reduced to 0 hit points. If it succeeds its hit points are reduced by half of their current amount. Once you use this feature you can't use it again until you finish a long rest.