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

Beginning at 3rd level, you gain the Poisoner Mastery Property and may use it on all weapons that you have Mastered even if they cannot normally have that property. When you use your Cunning Action, you can apply poison to your weapon in addition to the action you took.

When you reach 3rd level, you finish preparations for your false identity. Your false identity has a history, profession and affiliations appropriate to it. It cannot be an identity that already belongs to someone else. For example, you might have acquired 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.

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

You may spend seven days and 25 gp to create another false identity, allowing you to switch between any of your identities or your true self.

At 6th level, the poison you create with your Poisoner Mastery Property ignores resistance and immunity to poison damage or the Poisoned condition. In addition, you now regain spent uses of your Poisoner Mastery Property on a short rest.

At 9th level, you gain the ability to unerringly mimic another person’s speech, writing, or behavior. You must spend at least a minute studying any of these three components, listening to speech, examining handwriting, or observing mannerisms. If you are privy to more than one of these aspects during the minute you can mimic as many as you observed.

Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have a +10 on any Charisma (Deception) check you make to avoid detection.

At 13th level you now ignore resistance and immunity to poison damage as well as the poisoned condition for any poison that you administer. If a creature had no immunity or resistance to the damage it instead gains vulnerability.

Starting at 17th level, you become a master of instant death. When you attack and hit a creature that is surprised or administer poison to it, it must make a Constitution saving throw against your Mastery Save dc. On a failed save, double the damage of your attack or poison against the creature.