Mann Co. Spy

Spies are experts in infiltration and assassination. Very few ever see the unmasked face of a Spy, even supposing you become close enough allies to even see the masked face of a Spy. They imitate the appearances of those you know, and just when you least expect, you're face down in the dirt with a knife in your back. Spies are puzzles, wrapped in enigmas, shrouded in riddles. You never know who might be a Spy. They will breach your defences, assassinate your colleagues, and the worst part... He could be any one of us. He could be in this very room! He could be you! He could be me! He could even be-

- I made this subclass based on the Spy from the game Team Fortress 2. I do not own Team Fortress 2, Valve, or any of their associated trademarks. Avatars from reddit and tf2wiki.- 

Backstab

You have mastered the art of striking your enemy where they least expect.

At 3rd level, you deal critical damage on a 19 or a 20 when behind your enemy.
Additionally, you have advantage on attack rolls with simple or finesse melee weapons if you are behind your enemy and hidden from them. 

Spytron 3000

You easily disguise yourself as your enemy to attack them from behind.

At 3rd level, gain the use of Disguise Self at will. When casting Disguise Self this way, you may also change your voice. 
You may use your Cunning Action to cast Disguise Self. If you are disguised as an ally of a hostile creature, and this creature didn't see you cast Disguise Self, you may use a bonus action to make a Deception check contested by the hostile creature's Insight check. If you are successful, you are counted as hidden from this creature, even if the creature can see you, while Disguise Self lasts or until you attack the creature. 

Invis-Watch

You have achieved a way to render yourselves invisible.

At 3rd level, gain the use of Invisibility, which you can cast at will without any material components. When cast this way, you may only target yourself, and you are unable to attack or cast a spell until Invisibility ends. 

Eye for Weakness

Fool your enemies into trusting you.

At 9th level, when under the effects of Disguise Self and you are disguised as an ally of a hostile creature, you may spend an action to learn the hit point maximum and current hit points of one enemy creature you can see. You may use this ability once per long rest. 

Cloak and Dagger

Your cloaking ability has increased in complexity.

At 9th level, gain the use of Greater Invisibility without the need for material components. When cast this way, you may only target yourself. You can cast this spell this way once per long rest. 

Electro-Sapper

Learn how to draw the energy from artificial enemies. 

At 13th level, as an action, you may touch a creature in the construct creature type. Until the end of your next turn, this creature is stunned. Additionally, it becomes vulnerable to any damage it is not resistant or immune to. This effect ends early if the creature takes damage from one of its allies. 

You may use this ability once per short rest. 

Spycheck

Your senses are honed to detect rival spies. 

At 13th level, you can see the true form of any creature concealed by Illusion or Transmutation magic when they are in direct line of sight and within 30ft of you. 

Dead Ringer

You have mastered the ability to fake your own death. 

At 17th level, when you take damage, you may use a reaction to instead negate this damage. Conjure a fake corpse of yourself in the same space as you and become invisible until the end of your next turn. The corpse has the same physical appearance, clothes, and non-magical equipment as you. This invisibility ends early if you make an attack or cast a spell that deals damage. The fake corpse remains for 1 hour, after which it and all its equipment will crumble into ash. Every creature who witnesses this believes you are dead. 

You must use this ability after you discover if the attack hits, but before you discover the damage roll. You may use this ability once per long rest. 

Superior Cloaking

Your deceptive technology outmatches even the strongest Divination magic.

At 17th level, when under the effect of Disguise Self, creatures with Truesight will be unable to see through your illusory disguise, and Detect Magic will not reveal your disguise as Illusion magic to the caster.
When under the effects of Invisibility from your Invis-Watch ability, you are undetectable through Truesight or any means of magic short of a Wish spell while you are not moving. 
When under the effects of Greater Invisibility from your Cloak and Dagger ability, you are completely undetectable through any magical means short of a Wish spell.