The signature concealed weapon used by assassins in Assassin's Creed is called the Hidden Blade.
The Hidden Blade is a spring-loaded blade worn along the forearm. In the games, an Assassin can extend it quickly for a close-range strike and retract it to keep the weapon concealed. It became one of the franchise's most recognizable pieces of equipment.
Different games modify the design and add functions. In Assassin's Creed II, Leonardo da Vinci improves Ezio's equipment, while later protagonists receive versions with features such as poison delivery, a second blade, or a mechanism for launching a projectile. These variations do not change the weapon's standard name.
The series also uses other named tools, including the rope dart, smoke bombs, and the Phantom Blade. Those are separate weapons or devices, not alternate names for the Hidden Blade. The weapon's fictional history is linked to the Assassin Brotherhood's emphasis on secrecy and precision.