An assassin is the master of death. they are experts of finding there target and eliminating them by any means necessary. Whether by creeping through shadows, toxins, or sifting through a crowd. There is no safe place from a trained assassin.

Bonus expertise

When you choose this archetype at 3rd level, you gain expertise with the disguise kit and the poisoner’s kit.

Assassination

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. When you have advantage on a target and hit them with a melee attack you may change that hit to a critical hit. You may use this feature a number of times per day equal to your proficiency bonus.

Flee the scene

Starting at level 9, when you kill a creature within 5 feet of you with a melee attack, you can immediately use a cunning action as a free action. If you take any other action or bonus action first, you cannot use the cunning action. 

Master of poisons

At 13th level, you gain mastery of your poisoner's kit. Any poison created by you with a poisoners kit will ignore resistance to poison damage and cause disadvantage on saving throws against the poison.

This ability is improved at 17th level. At 17th level none can ignore your deadly concoctions, your poisons now treat immunity as resistance for poison damage. 

Master Assassin

Starting at 17th level, you become a master of instant death. When you attack and hit a creature with your assassination ability, 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.