Infiltrator

Regarded as the pinnacle of many Underdark societies, when it comes to serving noble houses, criminal gang organizations, or warring businesses, infiltrators are the key to victory. Skilled spies and assassins that hide in the shadows, they remain hidden until necessary action is needed. While they usually gather intelligence for themselves or their masters from a safe distance, they should never be underestimated if encountered directly. 

Infiltration Skills

3rd-level Infiltrator feature
Your enhanced spy abilities allow you to sneak, move, and steal with phenomenal skill. You gain a +2 bonus to ability checks with Dexterity (Acrobatics), Dexterity (Stealth), and Dexterity (Sleight of Hand). 

Follow-Up Attack

3rd-level Infiltrator feature
Your quick reflexes allow you to follow-up an ally's successful attack with an attack of your own. When a creature within 60 feet of you that you can see is hit by an attack made by one of your allies, you can use your reaction to make a weapon attack against that creature. 

You can use this feature a number of times equal to your proficiency bonus and regain all expended uses when you finish a long rest. A use of this feature is only expended when you hit a creature. 

Defensive Reflexes

9th-level Infiltrator feature
You have honed your reflexes to avoid being hit by your enemies. When you use your Uncanny Dodge feature, you can choose to take no damage instead of taking half damage.

You can't use this feature more than once during a combat encounter, and you can't use it at all outside of combat. 

Combat Ready

13th-level Infiltrator feature
When you roll for initiative and get a result of 20 or higher, you can make one melee or ranged weapon attack before the first creature in the initiative order takes its turn. 

Intiltrating Assassin

17th-level Infiltrator feature
While there are no friendly creatures in sight or within 120 feet of you, you gain advantage on attack rolls and ability checks with Dexterity (Acrobatics), Dexterity (Stealth), and Dexterity (Sleight of Hand). 

In addition, if you name a specific creature as your assassination target at least 10 minutes before encountering that creature and surprise that creature when you attack them, your first attack against them is an automatic critical hit.