Executioner

Whether you worked as a trained assassin in a thieve's guild or as the royal execution under a monarch, killing has been a part of your life for as long as you can remember.  Utilizing your weapon of choice, be that a dagger, an axe, or a pair of crossbows, you are not afraid to spill blood in a quick and efficient manner.

While an efficient killer in any situation, Executioners work best when their foes are already on the back foot and marked for death.  With a quick slash of a blade or the firing of a bolt, those who are bloodied and close to death find it quick to come when you are around.

What brought you into this line of work? Were you trained to do so from a young age by some assassin's guild? Are you a servant of a god of blood and death, tasked with bringing low those who would oppose them? Or are you simply someone who enjoys the scent of blood on their hands?

Execution

Starting at 3rd level, you have learned to take advantage of the moment a creature's life force begins to wane in order to deliver a fatal strike.  When a creature within 30ft of you is reduced to half its hit point maximum or less, you can use a reaction to teleport within 10 feet of the creature and make an attack against it.

Slayer's Vigor

Also at 3rd level, you relish in the scent of blood and the feeling of bodies dropping at your feet.  When you reduce a creature to 0 hit points, any damage you dealt past what reduced to the creature to 0 is transferred to you as temporary hit points.  You can gain these temporary hit points once, and regain the ability to do so when you finish a short or long rest.

Additionally, you gain an additional way to use your Sneak Attack.  You don't need advantage on the attack roll to use your Sneak Attack against a creature if they are at half their hit point maximum or less.

Sharpened Instruments

Starting at 9th level, you've learned to inflict harm unto others with greater effect.  When you roll a 1 on your Sneak Attack die you can instead treat the number rolled as a 2.  Also, a number of times per day equal to your proficiency bonus, you can change the damage type an attack to Force damage as you assure your strikes will remain deadly when you need them to most.

Chain Kill

By 13th level, you have practiced the art of killing enough to do so multiple times in quick succession with ease.  When you reduce a creature within 10 feet of you to 0 hit points, you can make an additional attack against another creature within range.  If that attack hits, you deal Sneak Attack even if you already used it this turn.  You can use this ability a number of times equal to your proficiency bonus, regaining all uses when you finish a long rest.

The minimum number you can roll on a Sneak Attack die increases to 3.

Finishing Blow

When you reach 17th level, you gain insight into how best to bring down your targets.  Once per short rest, when you would make an attack roll against a creature, you can instead choose to automatically treat the roll as if you had rolled a 20 on the die.