Overview
Blades in the Dark is a heist-themed tabletop roleplaying game system about a crew of daring misfits on the streets of an industrial fantasy city. Published by Evil Hat Productions in 2017 and designed by John Harper, it introduced the highly influential
rules engine. BitD revolutionized the heist genre in tabletop gaming by pushing players straight into the act, eliminating the "planning phase".
The system it loosely evolved from is
Apocalypse World
Description
In Blades in the Dark, players are part of a criminal organization in Doskvol, a grim Victorian era city. The city is protected by a wall of lightning, to keep any threat on the outside. Players create their characters as either smugglers, thieves, assassins or cultists and explore the criminal underworld, perform dangerous heists, manage gang warfare and grow their criminal organization.
The game focuses heavily on narrative momentum and failing forward. Characters are competent but flawed, constantly accumulating stress, trauma, and heat from the law as their gang rises to power.
System Overview & Key Features
Action Roll (d6 Dice Pool)
The core mechanic uses a pool of six-sided dice(d6) based on a character's action rating. The highest die result will determine the outcome of the action. 6 is considered a full success, 4-5 is a partial success with a consequence, and 1-3 is a failure. Rolling multiple full successes is considered a critical success.
Position and Effect
Before every roll, the Game Master explicitly states the character's Position (Controlled, Risky, or Desperate) and Effect (Limited, Standard, or Great). This transparently communicates the exact stakes and potential fallout of the action before the dice hit the table.
Stress and Resistance
Characters have a Stress track. Players can spend Stress to give themselves for extra dice when rolling. They can also use it to assist an ally, or fuel their special abilities. Players can also spend Stress when facing any situation or consequence the GM throws at them, this makes the characters very skilled, but forces them to consider when to spend it.
The Flashback Mechanic
Instead of spending hours planning a heist, the crew simply chooses a target, picks a detail, and jumps right into the action. Later, they can use Stress, to envoke a flashback moment, where they explain how they had already planned for such a situation, or had "solved" it beforehand.
Progress Clocks
The game uses circular progress tracks called Clocks to visually represent approaching danger or the progress of a complex task. A clock might represent the alert level of the city watch, the structural integrity of a safe vault, or the patience of a rival gang leader.
The Crew as a Character
The criminal organization the players run has its own character sheet. The organization gains experience, levels up, claims territory, and unlocks special abilities, ensuring that the gang's rise to power is just as mechanically significant as the characters'
Additional links
evilhat.com 
https://evilhat.com - Official Evil Hat Productions website
bladesinthedark.com https://bladesinthedark.com - Official System Reference Document and rules database