Updated assassin

Bonus Proficiencies

When you choose this archetype at 3rd level, you gain proficiency in the Persuasion and Deception skills if you don't already have it. Your proficiency bonus is doubled for any ability check you make that uses either of those proficiencies.

Assassinate

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You can add your proficiency bonus to initiative rolls, and the first weapon attack you hit against a creature in combat is a critical hit. 

Infiltration Expertise

 Starting at 9th level, you can unfailingly create false identities for yourself. During a long rest you must spend 5 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.

 

Starting at 9th you gain proficiency with the Poisoners kit if you don't already have it. If you  are already proficient then your Proficiency is doubled. Additionally you can spend 5GP to create a special concoctions that will give you an additional edge over your enemies. You can craft one of four brews over the course of a long rest, the number of brews created is equal to your half your proficiency bonus (rounded down). The cocktails can be coated on any blade or on 10 pieces of ammunition as a free action, the concoctions last for one minute. A creature who passes the saving throw is immune for the next hour. A creature struck by a critical hit makes it's saving throw with disadvantage. 

Other creatures coat their equipment as normal. 

DC= 8+Prof modifier+Dexterity Modifier

Basilisks Gaze: This powerful brew attacks the nerves of its victims locking them in place for a short period of time but while powerful creatures quickly grow inmune to it. On a hit the creature makes a CON saving throw on a failed save the creature is paralyzed until the end of your next turn. Once used a new brew must be applied over the blade as the entire dosage is absorbed by the target.

Daemoninc Darkness: On a hit the creature loses control of all their sences, the creature must make a CON save for one minute the creature is blinded and deafened and loses all additional senses they have. (blindsight, tremorsence, ETC) The creature can remake their saving throw at the end of each of their turns.

Touch of Erosion: A dark cocktail that enters the target eating them from the inside out until nothing is left behind, creatures killed by a blade coated by this brew desintagrate leaving only their equipment behind as all biological mass is eaten by the spell. On a hit the creature is forced to make a CON save, on a failed save the creature takes 2D6 acid damage or half as much on a successful one, the damage goes up as you take levels on this class. (3D6 at 13th), (4D6 at 17th level) and (5D6 at 20th) A creature is not able to become inmune to this brew and will continue to recieve the effects of being struck by it.

Impostor

At 13th level, you gain the ability to unerringly mimic another person’s speech, writing, and behavior. You must spend at least 10 minutes 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.

You can assume the identity of any creature you have studied when using Impostor. 

Additionally you gain an uncanny ability to mask your true identity, having studied the abilities of changelings and dopplegangers. you learn the disguise self spell and can cast it a number of times equal to your proficiency modifier. You regain all uses after a long rest.

 

Death Strike

Starting at 17th level, you become a master of instant death. When you attack and hit a creature that is surprised or score a critical hit , 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.