Assassin.

You focus your training on the grim art of death. Those who adhere to this archetype are diverse: hired killers, spies, bounty hunters, and even specially anointed priests trained to exterminate the enemies of their deity. Stealth, poison, and disguise help you eliminate your foes with deadly efficiency.

Level 3 Feature:

Tools of the Trade.

When you choose this archetype at 3rd level, you gain proficiency with the disguise kit, the poisoner's kit and one martial weapon of your choice. Your proficiency bonus is doubled for any ability check you make that uses either the disguise or poisoner's kit, and any poison you make deals extra damage equal to your dexterity modifier.

Assassinate.

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You add your proficiency bonus to initiative rolls and you have advantage on attack rolls against any creature that hasn't taken a turn in the combat yet.
In addition, any hit you score against a creature that is surprised is a critical hit.

Level 9 Feature:

Marked For Execution.

At 9th level, you are able to single out your targets both in and out of combat, analysing their movements and behaviour patterns to size them up.
As a bonus action, you can mark one creature you can see for execution. All sneak attacks you make against this creature for the next minute use d8s instead of d6s. This increases to d10s at 16th level. You can only do this once per combat encounter. Additionally, you can spend 1 minute closely observing a creature you can see and hear. Doing so grants you advantage on all Charisma checks you make to influence it in the next 24 hours.

Level 13 Feature:

No Escape.

At 13th level, you are able to shut down any attempts made by your quarry to escape, ensuring none survive to compromise your goals.
Whenever you hit a creature with less than half of its hit points remaining with a weapon attack or reduce a creatures hit points below half with a weapon attack, their speed is halved and they lose the ability to take the disengage action, teleport themselves, or travel to another plane of existence until the end of their next turn.

Level 17 Feature:

Death Strike.

Starting at 17th level, you become a master of instant death. When you attack and hit a creature that is surprised, it must make a Constitution saving throw (DC 8+ your Dexterity modifier + your proficiency bonus). On a failed save, double the damage of your attack against the creature.