Bonus Proficiencies
When you choose this archetype at 3rd level, you gain proficiency in either Deception, Investigation, or Performance.
Assassinate
Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn't taken a turn in the combat yet.
You also 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 you score a critical hit against them. All the other rules for Sneak Attack still apply to you.
Careful Strike
Starting at 9th level, you can steady your hands for a well-placed blow. You can use your action to target an enemy that you can see; the next attack against that enemy that you make scores a critical hit on a 18-20. If you do not make this attack after 1 round, this benefit goes away.
Impostor
At 13th level, you gain the ability to unerringly mimic another person's speech, writing, and behavior. You must spend at least three hours studying these three components of the person's behavior, listening to speech, examining handwriting, and observing mannerisms.
Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have advantage on any Deception check you make to avoid detection.
Death Strike
Starting at 17th level, you become a master of instant death. When you score a critical hit against a creature, 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.