While the Assassin's Creed franchise has depicted various weapons over the years, from guns to powerful Swords of Eden, the most iconic weapon is the Hidden Blade. It is the defining attribute of an Assassin, with the formula only being slightly meddled with over the years. Despite this, and perhaps because of this, the Hidden Blade has a longer and more vibrant history in Assassin's Creed than one might imagine.
Assassin's Creed Odyssey, the oldest chronological entry in the franchise, explains the very first instance of a Hidden Blade use. It was used by Darius to assassinate King Xerxes around 500 BCE, while Alexios or Kassandra would use a type of "Proto-Hidden Blade" in Leonidas's Spear. Eventually, the Hidden Blade would be passed on to Aya and then to her husband, Bayek.
An early Assassin tradition, prominent in the original Assassin's Creed game but dismissed well before Assassin's Creed 2, was the removal of the finger for the use of the Hidden Blade. This began when Bayek accidentally severed his ring finger with his own hidden blade during the events of Assassin's Creed Origin. About 1100 years or so pass between Origins and the first game, but by then, the Hidden Blade was an operative part of the Hidden Ones as they transitioned into the Assassins.
It wasn't until then and after Altair's ascension to Mentor that the Hidden Blade was redesigned, but this wasn't just the spark of creative invention. The Apple of Eden actually inspired Altair to make the change, so later assassins would not need to lose a finger. And then a couple more hundred years would pass, with the Hidden Blade receiving further updates in the 15th century.
Prior to this, a wrist flick was required to use the blade, but the mechanism had since been changed to allow it to extend in a variety of ways, though the wrist flick was still a common method of unleashing the blade. Assassin's Creed 2 also saw the addition of a second Hidden Blade to the protagonist, which had two reasons: one for lore, one for more rudimentary purposes. Wielding two Hidden Blades became a method of identifying a Master Assassin, while it was added to the game so that the Assassin's Creed 2 protagonist could have 2 Hidden Blades. Art for the next game initially portrayed Ezio with three Hidden Blades for this same purpose, before it was discarded.
Inspired by Altair's writings, Leondaro Da Vinci also created for Ezio quite a few variations of the Hidden Blade such as the Hidden Gun and Poison Dart launcher. These never became mainstays in the brotherhood, it seems, as they were quickly replaced with more viable weapons in later years. Their addition in Ezio's trilogy seems to have been for varied gameplay purposes, meaning they didn't have much weight on the lore, unlike the finger modification.
Over the next two hundred years or so, individual brotherhoods adapted their own style of Hidden Blade for their Assassins. These included, but are not necessarily limited to, the Ottoman Assassins' Hook Blades, the Indian Assassins' extra blades, and the Chinese Assassins' footblade. Smaller changes also occurred during this time period on, which include wider blades as seen by the Carribean and Colonial Assasins, and a pivot blade that operated secondarily as a dagger also emerged. Utilizing two hidden blades was also no longer relegated to just a Master Assassin.
Moving into the 21st century, the Hidden Blade fell out of popularity, though it is not fully explained why. It would seem likely because the Templar vs. Assassin conflict had outgrown simple methods of winning, even temporarily, though Assassins like Desmond Miles put it to good use. Still, it was still used in some regions like Japan and India, where the Indian Assassins also used explosives attached to this iconic weapon.
While the history of the overaching Hidden Blade may be brief, it's clear that it's played a huge, evolving role in the ongoing Assassin vs. Templar conflict. It remains to be seen if the next entry, rumored to be about vikings and titled Assassin's Creed Ragnarok, adds more to this history, but it doesn't seem likely to be big overarching lore elaborations. This is because, chronologically speaking, it would happen prior to the first game, so the protagonist is likely to be another poor soul to lose their finger in this moment of the weapon's history.
Assassin's Creed Ragnarok is rumored to be in development.