Hidden Blade
The Hidden Blade is Altaïr's most iconic and precious weapon, mainly used for stealth assassinations. A successful assassination with this weapon is always fatal.
You are given this weapon at the start of the game. After losing this weapon in your demotion near the beginning of the game, it is returned to you so that you may perform your first assassination on Tamir the Merchant in Damascus.
It is worth noting that Altaïr's descendant Ezio Auditore de Firenze, who you play as in Assassin's Creed II, will possess two of these blades, whereas Altair has only one.
Use
This weapon is mostly used for stealthy, low-profile assassinations upon soldiers. An unaware soldier may be stabbed in the neck or the stomach without drawing too much attention. When stealth is not a priority, Altaïr may perform a high profile assassination. This involves jumping upon an opponent, causing his victim to fall on the ground. Then the blade is triggered, stabbing the opponent in the throat.
Both low and high profile assassinations may fail. A guard wary of your actions may catch your arms when you attempt the kill and throw you back. During combat, it is impossible to assassinate your enemies unless they have been thrown onto the ground or are fleeing from you. You will always be stopped.
Be aware, of course, that you will lose a hefty bit of Synchronization if you assassinate innocent civilians with the hidden blade, just as with any other weapon.
Role in investigations and missions
Assassination missions
The Hidden Blade is also used for one of the investigations given to you by informants. You are asked to stealthily assassinate a number of targets. If any guard even once confronts you, the mission is failed. If you attack the targets or the guards with fists, the short blade, the longsword, or any other high profile weapon, you will fail the mission. Because of this, you may ONLY use the throwing knives or the Hidden Blade.
Interrogation missions
During Interrogation missions, after successfully interrogating the target, Altaïr is forced to kill them, so they can not reveal his plans to the enemy. He will bury the blade in their stomach to kill them, then gently lower their body onto the ground so as not to draw further attention to himself. Occasionally, other assassinations performed with the Hidden Blade play out in the same manner.
Save Citizen missions
In the optional Save Citizen missions to save citizens from soldiers threatening to kill them, this weapon will come in handy. Successfully and stealthily used, you might take out three of the soldiers before they notice you, draw their weapons, and engage you in a swordfight. If they notice your tactic, however, you'd best be prepared and draw either your short blade or sword for a swordfight.
Assassinations
The hidden blade is used in every storyline assassination you perform. Even if you begin by attacking the enemy with a sword, you will always finish them off with the hidden blade before it cuts to the death cutscene.
Mechanism
When not in use, the blade is hidden in a gauntlet on Altaïr's lower left arm. Altaïr triggers it via a ring on his pinky finger. This causes a spring system to eject the blade, hopefully into the neck or stomach of an enemy.