Your job is to get into and out of places as fast as possible

Sharp as a Tack

When you choose this archetype at 3rd level, you add your Proficiency bonus to your initiative 

Also as part of this ability the damage that you deal with a dagger increases to a d6 at 5th level, d8 at 11th level, and d10 at 17th level of this class. 

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.

As a bonus action on your turn, you can Mark you enemy for assassination. Anytime you would deal sneak attack damage to that creature you can instead choose to use a portion of those dice to heal yourself instead. 

Getting the Drop

Starting at 9th level, you have gained the ability to always get the drop on your enemies and to end them rightly. Whenever you make a initiative roll, treat a roll of 9 or lower on the d20 as an 10.

Additionally, as part of your initiative roll, you can move up to your movement speed, and either make one weapon attack or take a "Cunning Action".

The grim reaper

At 13th level you gain the ability to momentarily absorb the knowledge of death itself. When you take this action, you gain a fly speed equal to your walking speed for the next 10 minutes. You also critically strike on a 19 or 20. 

If you critically strike you can expend any number of hit die to pump into the enemy and add twice the amount rolled using those hit die to the damage to the creature. You can only do this a number of times equal to your proficiency modifier.  

Grim 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.