What is the name of Altaïr's signature concealed weapon in Assassin's Creed?

Altaïr's signature concealed weapon in Assassin's Creed is the Hidden Blade.

The Hidden Blade is a spring-loaded assassination weapon worn beneath an Assassin's bracer. It extends toward the hand and lets its user strike discreetly at close range. In the original game, Altaïr uses it as one of the defining tools of the Assassin Brotherhood.

The weapon is fictional, but the series presents it as part of a long tradition passed between Assassins. Several games connect its operation with the removal or alteration of a finger, although later lore changes how that practice is understood. The design also evolves: later protagonists use paired blades, poison mechanisms, retractable variants, and other modifications.

The Hidden Blade is not the same as the Phantom Blade introduced in Assassin's Creed Unity. The Phantom Blade is a crossbow-like wrist weapon, whereas the Hidden Blade is primarily a concealed melee assassination tool.