Guildmaster's Shadow
Traits
Evasion. On a DEX save that deals half damage on a success, this creature takes no damage on a success and half on a failure.
Assassin's Mark. During a short rest, chooses a target. Attacks that target with Advantage, and Sneak Attack damage against it increases by +2d6.
Sneak Attack (1/Turn). When attacking with Advantage or when a target has an ally within 5 feet of it: 28 (8d6) extra damage.
Urban Camouflage. In a crowd, shadow, or other urban cover, can take the Hide action as a Bonus Action.
Actions
Multiattack. Makes three Poisoned Blade attacks.
Poisoned Blade. Melee Attack Roll: +10, reach 5 ft. Hit: 10 (1d8 + 6) Piercing damage plus 14 (4d6) Poison damage, and the target makes a DC 16 Constitution save. Failure: The target has the Poisoned condition (1 minute). The target can repeat the save at the end of each of its turns.
Throwing Knife. Ranged Attack Roll: +10, range 20/60 ft. Hit: 10 (1d6 + 6) Piercing damage plus 7 (2d6) Poison damage.
Bonus Actions
Cunning Action. Takes the Dash, Disengage, or Hide action.
Reactions
Uncanny Dodge. When hit by an attack it can see, halves that attack's damage.
How to Use
The head of the assassins' guild. It closes in unseen, marks a target (usually the Wizard) with Assassin's Mark, and erases them in a single turn:
3 attacks × (10 + 14) + Sneak Attack (28 + 2d6 from Mark) = ~110 damage
Key
Not See Invisibility / Truesight — it isn't invisible, it's hiding. High Passive Perception, area spells (Evasion still leaves half damage), or forcing it into open ground (Urban Camouflage stops working).