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 precise strikes help you eliminate your foes with deadly efficiency.

Relying on expertly woven movements between each opportunity, you can act fast, and faster, making use of your entire skillset in the split seconds of conflict. Your quick wits and sense of momentum carry you to act with extraordinary focus, bringing sharp metal to vulnerable points in the blink of an eye.

Bonus Proficiencies

When you choose this archetype at 3rd level, you gain proficiency with the poisoner’s kit and your proficiency bonus is doubled for any ability check you make that uses this proficiency.
Additionally, whenever you hit with a Weapon Attack and use Sneak Attack, if that attack had a poison applied to it, you can choose to replace the poisons save DC with your own DC (8 + Your proficiency bonus + Your Dexterity modifier) as you strike the optimal position for the poison to take effect.

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

Quick Wits

Starting at 9th level, you can reliably act fast. When you roll for initiative, you have advantage on the roll, and you add your proficiency bonus.
Additionally, while rolling for initiative, but before a creature takes a turn, you can choose to add a bonus equal to your proficiency bonus to your initiative roll. This bonus to your initiative expires at initiative count 0. You can use this feature twice, and even on the same roll, and regain all uses upon finishing a long rest.

Deadly Momentum

Starting at 13th level, you gain the ability to make expert use of distractions and diversions to act further in another window of opportunity.
If an enemy creature within 30 feet of you is incapacitated in the first round of combat, you may take one additional turn that round. This turn occurs after all other creatures have acted. While taking this extra turn, if a creature could not act freely due to being incapacitated or some other effect that made it waste its turn or force it to act in a way it normally wouldn't - such as being forced to take the Dash action and flee, waste their action doing nothing, or any effect controlling their actions originating from a creature hostile to that creature - it is considered to be surprised and to have not taken a turn yet.

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.