Improved Assassin

Good at going fast and getting crits. 

Quick to Act

When you choose this archetype at 3rd level, you make initiative checks with advantage, and you add your proficiency bonus to your initiative roll.

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 has not yet taken a turn is a critical hit.

Deadly Poison

Starting at 9th level, you gain the ability to mix and apply poisonous substances as if you were a master alchemist. During every long rest, you are able to create poisons that increase in power as you increase in level, as shown in the Deadly Poisons chart. You can use an action to apply poison to a blade, which lasts for one hour, and deals additional poison damage on-hit.

Deadly Poisons	Poison Damage
9th	2d6
10th	2d6
11th	3d6
12th	3d6
13th	4d6
14th	5d6
15th	6d6
16th	6d6
17th	7d6

In addition, when you reach 12th level, you can create poisons that instead deal Necrotic damage, and when you reach 15th level you can create poisons that instead deal Acid damage.

Hitman’s Darkness

At 13th level, you gain the ability to cast the spell Darkness. You may cast this spell a number of times each day equal to your dexterity modifier. Whilst you reside in this Darkness, you are completely invisible and silent, even to creatures that can see through magical darkness. (They simply do not notice you are there).

Death Strike

Starting at 17th level, you become a master of instant death. When you attack and hit a creature that has not yet taken a turn in combat, 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.