True Assassin

Those who devote themselves to the underground side of the community have to develop many different kinds of skills to survive. This saying couldn't ring any more true for those that take on the profession of assassination. Whether lying in wait for the chance to strike from afar, to blending in with the crowd to become invisible and closer to your target, the assassin has quite a bit of training to help them make a clean kill and a clean getaway.

Bonus Proficiencies

When you choose this archetype at 3rd level, you gain proficiency with the disguise kit and the poisoner’s kit. If you already have proficiency in either make it expertise 

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. A number of times per day equal to your proficiency bonus you can turn any hit into a critical hit. This ability can only be activated when you have advantage on the attack roll

If this attack kills the target creature you may use your reaction to use cunning action 

Infiltration Expertise

Starting at 9th level, you can unfailingly create false identities for yourself. You must spend seven days and 25 gp to establish the history, profession, and affiliations for an identity. You can’t establish an identity that belongs to someone else. For example, you might acquire appropriate clothing, letters of introduction, and official-looking certification to establish yourself as a member of a trading house from a remote city so you can insinuate yourself into the company of other wealthy merchants.

Thereafter, if you adopt the new identity as a disguise, other creatures believe you to be that person until given an obvious reason not to.

Poison crafter

When you gain this class feature at 5th level you are able to create a number of poisons up to your proficiency bonus on a long rest. These poisons are considered large enough for a single dose. Any poison that is not used becomes ineffective after 24 hours or at your next long rest. A creature afflicted with one of these poisons must make a Constitution saving throw.

 Save DC = 8+ proficiency bonus(poisoner's kit)+ wisdom modifier

On a failed save the creature is takes 2D8 damage poisoned for 1 minute. On a success the creature takes no damage and isn't poisoned.

None the wiser

You gain advantage on Stealth (sleight of hand) checks to apply poisons that are not administered by injury or inhalation 

Opportunistic poisons

Whenever you have a weapon or piece of ammunition coated in one poison created by you, and you deal sneak attack damage; The creature has disadvantage on the saving throw to resist the poison. Also your poisons ignore resistance to poison damage.

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 Charisma (Deception) check you make to avoid detection.

Death Strike

Starting at 17th level, you become a master of instant death. When you attack a creature and use your assassinate feature, 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.

If the weapon or piece of ammunition is covered in a poison crafted by you when you use this feature the damage is now doubled.

Creatures with immunity to poison are now considered resistant to poison damage. This ability does not stack with opportunistic poisons.