Ninja

You train in the ways of the ninja.  becoming a master of shadows and stealth.  using this you gain advantage on Stealth and acrobatics, you gain access to 3 spells to help you succeed in your missions.  Arcane blade to add damage to your attacks twice per day.  disguise self at will and once per day you can use suggestion.. being able to use the shadows to benefit you it provides you with advantage on sneak attacks and if you surprise the person you get an automatic hit on your first attack.

at Level 9

you use your speed to provide bonus to your initiative to assist with the sneak attacks, as well as providing an easier chance to crit as well as more damage when you land a crit

at Level 13

you have the knowledge to manipulate your blades to cause major bleeding to your target.  this causes them to bleed every round

At level 17 you gain the ability to focus on vital points.  when performing a sneak attack they must make a constitution save. (DC 8 + your Dexterity modifier + your proficiency bonus) or become paralyzed.   On success they are not paralyzed  but still gets disadvantage on the next attack. 

Light on your feet

Due to you intense training. you have learned how to be very nimble gaining advantage on acrobatics and stealth

Shadow magic

Gain access to spells

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.

Lightning Speed

add double your dex modifier to Initiative 

Critical Mastery

You now crit on 19 and get 1 extra die on damage

Deep Cuts

All damage caused leaves a bleediing effect that causes 2d6 bleeding damage per turn

Vital Point Focus

At level 17 you gain the ability to focus on vital points.  when performing a sneak attack they must make a constitution save. (DC 8 + your Dexterity modifier + your proficiency bonus) or become paralyzed.   On success they are not paralyzed  but still gets disadvantage on the next attack.