The original design of the Hidden Blade, first seen during the Crusades, called for the user to cut off their own ring finger. This was not only seen as a necessity, but an act of commitment to the Assassin's Brotherhood, who have been at war with the Knights Templar for centuries. The most notable wielder of this early version of the hidden blade was master assassin Altaïr Ibn-La'Ahad.
However, the blade's mechanism was modified in the 13th century - as outlined in the Codex of Altaïr Ibn-La'Ahad - so as to no longer require such a sacrifice, and keep Assassins from being identified so easily.
Despite this, the tradition was not forgotten. From the Renaissance to at least the early 20th century, Assassins branded their left ring fingers during their initiation into the Order, as a sign of their devotion to the Creed.
The exact release mechanism of the hidden blade is unknown, and while there is a rotating wheel atop the rear portion of the mechanism's housing (one which turns upon the release or retraction of the blade), its function has not yet been identified.
High Middle Ages
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 hidden blade and other 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 an Assassin 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 would usually kill them so that they could not reveal his plans, and did so by burying his hidden blade into their abdomen.
Although the hidden 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, and thus did not require the removal of his ring finger.
The mechanism did not require a wrist "flick" to release the blade, thus the user could extend or 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.
Ezio Auditore inherited the hidden blade from his father 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 his ally Leonardo da Vinci to repair it. Leonardo was able to do so, though only 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, Ezio was equipped with a single hidden blade, though its bracer had been replaced. Later on, however, Leonardo 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 pick the locks of certain Lairs of Romulus, and operate the 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 late 19th and early 20th century, the hidden blade was still part of standard Assassin attire, and continued to be commonly used. Though the Assassins were already using guns, during close combat, the iconic hidden blade was occasionally utilized. The Russian Assassin Nikolai Orelov used his hidden blade during a battle with Tsar Alexander III, with which he was able to stab the Tsar in the abdomen.
By the late 20th century, hidden blades were less commonly used, and were mostly preserved for ceremonies. However, a few high-ranking Assassins still wore one for traditional purposes, such as Paul Bellamy. Daniel Cross was gifted with a ceremonial but functional hidden blade by the Mentor, which he swiftly put to use, killing the Mentor.
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, led by Warren Vidic.
Upon traveling to Monteriggioni, Desmond began using a bare hidden blade without a bracer, making it less visible on his arm, though at the cost of some defensive ability. 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.
High Middle Ages
As it was mainly used for stealth assassinations, the hidden blade could only be used in combat under certain conditions, and could not block any attack. Altaïr was only able to assassinate a guard who had fallen on the ground, or had become momentarily distracted.
Altaïr could use the hidden blade in a counter-attack; however, the time frame was extremely small, especially in comparison to that of the sword and short blade. Successfully intercepting an enemy attack enabled one of several different – and always lethal – counter-attacks, in which Altaïr stabbed his opponent in the chest, head, abdomen, foot, base of the skull, or spine.
Unlike other weapons, every counter-attack with the hidden blades resulted in an instant kill, with only the narrow timing window as a disadvantage. Unlike the sword or short blade, the hidden blades could counter attacks from nearly all types of guards, including Brutes, Seekers, and Papal Guards.
The hidden blade also gave Ezio more freedom in movement than any other weapon. For example, Ezio could easily strafe to either side while in combat, which was barely possible (except from very close range) with the sword or dagger. It also allowed Ezio to quickly flee from a battle without having to take the time to sheathe his weapon.
By his arrival in Rome, Ezio had gained a greater aptitude with the hidden blades. Aside from being able to perform more counter-attacks (due to his improved reaction time) he was now able to perform multiple instant kills after killing a single guard. To continue this "execution streak," each kill needed to be quickly followed up with another, without Ezio being hit in turn.
The first of these improvements was the addition of a secondary hidden blade, which was attached to the user's other arm, allowing Ezio to kill two guards at once, either on the ground or from the air. The second was a metal plate on top of the primary blade's bracer that allowed one to safely deflect enemy attacks with the blades.
Other Codex upgrades included the Poison Blade and Hidden Gun. The poison blade allowed Ezio to stab an enemy and leave them to die slowly, granting both a low-profile assassination as well as a distraction to kill or bypass other enemies, while the hidden gun allowed Ezio to make a loud but assured kill from a distance.
Additionally, both blades could be supplemented with various vambraces, which acted as protective armor.
Leonardo also designed and built the Hidden Bolt, a weapon that could fire small crossbow bolts, upon the request of Francesco Vecellio. As Francesco was only an apprentice at the time, Ezio had not allowed him to use the hidden gun, and thus Francesco had devised a similar, though lighter, alternative.
The Hookblade was a modification of the hidden blade that was adopted by the Assassins Guild of Constantinople. Unlike the other hidden blade advancements, it was attached to the user's secondary hidden blade, instead of the primary.
Upon his arrival in the city, Ezio also made use of it in both battle and travel. Primarily, it could be used during navigation in conjunction with ziplines, as well as allow one to perform "hook-and-runs" over enemies, and generally extend one's reach when climbing and leaping.
The hookblade also aided in combat by allowing a wider variety of attacks or counters, as well as permitting one to "counter-steal" or trip an enemy.
The first known instance of this was in the early 16th century, when Fiora Cavazza and Baltasar de Silva tailed the Assassins of Rome, in order to take note of their techniques. They eventually trained Il Lupo in the Assassins' ways, and equipped him with a Switchblade, a hidden blade variation whose blade was split into two parts, folding forward only when in use.
Lia de Russo also wore her own hidden blade, and though its mechanisms seemed to match that of the Assassins, it was worn on top of her forearm, rather than underneath.
In modern times, during the simulations hosted by the Animus Project of Abstergo Industries, every recruit was given access to a variation of the hidden blade. The virtual weapon was also worn under the left forearm, but its blade folded along two circular joints when not being used.