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. I have updated the abilities to bring them more in line with the other powerful rogue options and to make them more useful in a West march style server that is full of short encounters with very little room for Suprise rounds. I also believe that the slight benefits you get to disguises, when a check will most certainly be asked for anyways, is not enough for a full Subclass Feature. Finally, I have given more emphasis on poison with the hopes to make it more viable as well. 

Tools of the Trade

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.

You have resistance to poison damage due to your time spent building up your tolerance.  

You can use the bonus action granted by your Cunning Action to don a disguise or apply poison to a weapon or piece of ammunition.

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.
• Successful attacks against creatures that have not yet taken a turn in combat become critical hits. 
• You may add your Proficiency modifier to your Initiative rolls. 

Poison Expert

Starting at 9th Level, you gain the ability to craft longer lasting poisons and leverage them against your enemies.

When you create a poison in now lasts for a number of hits equal to your Intelligence modifier (Minimum 1).
You have advantage on attacks against creatures suffering from the poisoned condition.

Evanescence

Starting at 13th Level, you gain the ability to move about your environment seemingly without a trace.

You have advantage on ability checks you make to obscure your influence, such as creating false evidence or making an object appear untouched after interacting with it.

In addition, when you kill a creature or reduce it to 0 hit points, you can use your reaction to take the Dash, Disengage, or Hide action.

Death Strike

Starting at 17th Level, you become a master of instant death.

When you attack and hit a creature and deal sneak attack damage, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, double the sneak attack damage against the creature and the creature is stunned until the end of your next turn. (Your server/DM may want to limit this by including that this ability may be used a number of times per day equal to your proficiency bonus.)