Professional

Professionals can be master thieves, assassins, or secret agents. All that matters is they take pride in their work because they are the best at what they do. In their line of work reputation is everything, failure is unacceptable, and you never break a contract.

Mastery

Starting at level 3 you gain a number of mastery dice equal to the number of sneak attack dice you have. Your mastery dice are also D6's. You can use your mastery dice to add to certain rolls, depending on your specializations. You may use your mastery dice after seeing a roll, but before knowing the result. You may only add one mastery dice per roll, and you regain all your mastery dice after completing a long rest.

Specializations

Starting at level 3,  you can choose certain areas to specialize in. This determines what kind of rolls you will be able to add your mastery dice to. You have a number of specializations equal to you proficiency modifier.

Pickpocket: You may add your mastery dice to sleight of hand checks

Silver Tongue: You may add your mastery dice to persuasion checks.

Escape Artist: When escaping from restraints you may use dexterity rather than strength, and you can add your mastery dice to any dexterity skill check to escape restraints or from a grapple

Imposter: You may add your mastery dice to deception checks

Ghost: You may add your mastery dice to stealth checks

Treasure Hunter: You may add your mastery dice when you use thieves tools to open locks, and to perception checks to find secret doors

Dungeoneer: You may add your mastery dice to investigation checks to detect traps, and to checks to disarm traps

Killer: You may add your mastery dice to sneak attack damage. Even after improved mastery, you may only ever use on e mastery dice per roll on this specialization.

Ambusher: You may add your mastery dice to initiative rolls

Bounty Hunter: You may add your mastery dice to survival checks when looking for tracks or otherwise tracking someone, as well as strength checks when grappling

Enforcer: You may add your mastery dice to intimidation checks and saving throws against the frightened condition

Poisoner: When one of your attacks causes an enemy to make a saving throw against poison damage or the poison condition, you may use your mastery dice to subtract from their saving throw. Even after Improved Mastery, you may only ever use one mastery dice per roll on this specialization.

Improved Mastery

Starting at level 9,  you find ways to make your experience useful to your allies. When an ally within 60 feet of you makes a roll that coincides with one of your specialties, you may use your reaction to  add one of your mastery dice to their roll. You may do so after seeing the roll, but before knowing the result.

Also at level 9, you may add any number of mastery dice to your own rolls that coincide with your specialties.

Mental Discipline

Starting at Level 13, you have trained to shield your mind from outside influence in order to keep certain information secret. You are immune to the charmed condition, and no one is able to read your thoughts unless they know Thieves Cant.

Not until the job is finished

Starting at level 17, when you are reduced to 0 hit points but not outright killed, you go to 1 hp instead. In addition you may roll any number of your remaining mastery dice, gaining twice the amount rolled as temp hp. Once you've used this feature you may not use it again until you have completed a long rest.