Rogue: Assassin

You focus your training on the grim art of death. Those who adhere to this archetype are diverse: hired killers, spies, bounty hunters, and even specially anointed priests trained to exterminate the enemies of their deity. Stealth, poison, and disguise help you eliminate your foes with deadly efficiency.

Tools of the Trade

When you choose this archetype at 3rd level, you gain proficiency with the disguise kit and the poisoner's kit. Your proficiency bonus is doubled for any ability check you make that uses either the disguise or poisoner's kit, and any poison you make deals extra damage equal to your Intelligence modifier. Additionally, you can apply poison to your weapons and ammunition using a bonus action, rather than an action.

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 encounter yet. In addition, any hit you score against a creature that is surprised is a critical hit.

Hide in Plain Sight

At 9th level, your confidence in your skills allows you to work with fearless audacity. You have advantage on Dexterity (Sleight of Hand) checks to slip poison into food and drink, plant fake evidence on unsuspecting targets, and sneak hidden weapons past guards and inspections, as well as Charisma (Deception) checks to maintain a disguise or false identity.
In addition, you may add your Intelligence modifier to these checks.

One with the Shadows

At 13th level, you know how to remain hidden whilst attacking foes from the shadows. While hidden, your first attack does not immediately reveal your location. Make a Dexterity (Stealth) check contested by the target's Wisdom (Perception) check. On a success, you stay hidden.
You may use this feature a number of times equal to your Dexterity modifier, and you regain all expended uses when you finish 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, 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.