Arch Thief

Invisibility.  As result of the experiments of the thieves' guilds artificers, all four of the arch thieves are capable of temporary invisibility. (mostly for 15 mins.)

Knowledge of deception. Given the arch thieves experience, and knowledge of people, they can't be tricked through deception and they can't be persuated because of their loyalty towards the Nightingale. As a result of this, they can easily manipulate people. (Immune to deception and persuation, and they have advantage on charisma checks.)

Evasion. If the thief is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the thief instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Sneak Attack (1/Turn). The thief deals an extra 7 (2d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the thief that isn’t incapacitated and the thief doesn’t have disadvantage on the attack roll.

Multiattack. The thief makes two attacks with its rapier.

Rapier. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage.

Light Crossbow. Ranged Weapon Attack: +6 to hit, range 80/320 ft., one target. Hit: 7 (1d8 + 3) piercing damage.

Cunning footwork. Due to being fit to the thief lifestyle they are capable of moving with such elegance, and concentration that most people can hardly comprehend their speed, and almost nobody can predict their movements. (They can move through the battlefield in combat without the enemies' ability to react to them, similarly to the swashbuckler archtype for rogue, see in Xanathar's guide to Everything.)

Uncanny reflexes. As a result of their overwhelming experience they can react to anything even before it happens. (they can react to a thing before it happens, like for example if they are being hit, they can react to it before their damage is calculated.)