The iconic weapon of the Assassins, the Hidden Blade was a weapon used for both stealth assassinations and regular combat. It was their most versatile weapon, with multiple upgrades and forms of use.
It consisted of a retractable blade, usually in conjunction with a bracer, which could be discretely extended or retracted; making it a valuable tool for assassinations. It was commonly worn on the user's left arm.
The Assassin Altaïr Ibn-La'Ahad initially wielded a hidden blade, however, due to his failure in retrieving the Apple of Eden, and his breaking of all three tenets of the Creed, he was demoted to a Novice by Al Mualim, and was stripped of his weapons. At his first promotion to the second rank however, he earned back his hidden blade, along with his sword.
Aside from assassinations, the hidden blade was also used in a variety of investigations. For missions given by informants, wherein Altaïr was asked to stealthily assassinate a number of targets, he could only make use of his hidden blade or throwing knives in order to remain undetected. Additionally, after successfully interrogating a target, Altaïr was forced to kill them so they could not reveal his plans, and did so by burying his hidden blade into their abdomen.
Although the blade was used mainly for stealth, it could also be used in open conflict, but only for countering enemy attacks.
In his own time as an Assassin, Giovanni Auditore also wielded a hidden blade, albeit one that had been adapted to the change of the times. As outlined in the Codex, the revised blade had been designed by Altaïr so that the Assassins, who he intended to go undercover, could not be identified by their lack of a ring finger.
Furthermore, the mechanism did not require a wrist "flick" to extend the blade, thus the user could retract the blade without any wrist or hand movement. Additionally, as pictured, the weapon could be used with a clenched fist, although Giovanni is one of the few known Assassins to use it in that manner.
While there is a rotating wheel atop the rear portion of the mechanism's housing (one which turns upon releasing or retracting the blade), its function has not been identified, and there are no known schematics or explanations for the mechanism.
Ezio Auditore inherited the hidden blade from his father, along with his Assassin's robe, the night before the Auditore execution. However, the blade was broken due to Giovanni's last fight in Rome against Rodrigo Borgia and his men, thus Paola suggested that Ezio ask Leonardo da Vinci to repair it. Leonardo was able to do so, with the aid of the schematics detailed in the Codex.
Through other Codex pages, Leonardo was able to provide Ezio with a variety of upgrades; such as the addition of a second hidden blade. Ezio made use of these innovations for the majority of his assassinations, and was also able to wield them in battle.
Though Ezio possessed the same two hidden blades until 1500, during the Fall of Monteriggioni, he lost most of his weapons, and was only able to salvage the blade and bracer of his father. Upon his arrival in Rome, he was equipped with a single hidden blade, though its bracer had been replaced. Later on, however, Leonardo da Vinci was again able to reconstruct a secondary hidden blade from memory, with Ezio providing him with the money necessary for the raw materials.
Aside from using them in combat, Ezio also made use of his hidden blade to lockpick the entrances of certain Lairs of Romulus, and mechanisms within the Temple of Pythagoras.
Additionally, Ezio's apprentices were each equipped with a single hidden blade, as the right to use two hidden blades was only reserved for the most trusted of Assassins, or those who had earned the rank of Master Assassin. However, upon reaching the rank of Assassino, the apprentice was given a hidden blade with a bracer much like Ezio's. The apprentices were capable of all of the standard assassination techniques.
During the escape from the Assassin Hideout, Desmond Miles obtained a hidden blade and bracer that was almost identical to Ezio's first hidden blade. He later used that hidden blade to fend off the Abstergo guards who had come to capture him.
Later on, upon traveling to Monteriggioni, Desmond began using a bare hidden blade without a bracer, making it less visible on his arm. He also used it to operate mechanisms in the tunnels underneath the Sanctuary, as Ezio had once done.
Additionally, in the Animus Virtual Training Program developed by Rebecca Crane, Desmond was given the chance to hone his skills with the hidden blade. Such challenges included Stealth Assassinations and Flawless Kill Streaks using only the hidden blade.