The Assassin is an archetype that heavily favors stealth, infiltration and the use of poison to give them the upper hand in any engagement. Preferring to strike first and that strike to bring a swift end to the problem.

Proficiencies

Starting at 3rd Level, you learn the tools used by Assassins.

You gain proficiency with the disguise kit and the poisoner’s kit.

Your resourceful efficiency allows you to craft disguises and poisons at half the cost and time required.

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 a creature that hasn't taken a turn in combat yet.
• The first successful attack against creatures that have not yet taken a turn in combat becomes a critical hit. 
• You may add your Proficiency modifier to your Initiative rolls. 

Infiltration Expertise

Starting at 9th level, you can unfailingly create false identities for yourself.

You must spend one hour and succeed in a Deception or Performance check (DC Determined by the DM) 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.

Impostor

At 13th level, you gain the ability to unerringly mimic another person's speech, writing, and behavior.

You must spend at least one minute studying these three components; 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 successfully use your Assassinate ability on a creature, double the damage of your attack against that creature. It must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, it gains the poisoned condition until the end of your next turn.