"The Hidden Blade has been a constant companion of ours over the years. Some would say it defines us – and they would not be entirely wrong. Many of our successes would not have been possible without it."―Altaïr Ibn-La'Ahad's Codex, page 13
Iconic style, signature weapon. There's literally nobody who plays Assassins Creed without naming the presence of the Hidden Blade in both gameplay and narration. Throughout the franchise we've seen the weapons evolve just as much as the protagonists have. In the truest sense of the word, it's not an Assassins Creed game without the Hidden Blade. From the earliest times, the blade required a wrist flipping mechanisim for the weapon to work as well as the removal of one's finger; but now we've seen several updates and variations of it.
Several being the secondary Hidden Blade, Poison Blade and Hidden Gun. The Ottoman Hookblade wielded by both Yusuf Tazim and Ezio Auditore, helped in the process of travel as well as fighting - making it easier to zip through ziplines and provide the "hook and run" / "hook and throw". The Hookblade also aided in battle by allowing a wider variety of attacks or counters, as well as permitting one to "counter-steal" or trip an enemy.
As seen throughout the franchise Chinese Assassin, Shao Jun uses a modified Hidden Blade on the sole of her shoes. While Indian Assassin, Arbaaz Mir used a Hidden Blade that would extend into three blades. The Phantom Blade to be seen in Assassins Creed: Unity is another version that we'll be seeing in the near future.
Needless to say, you're not an Assassin without a Hidden Blade. So yeah, kind of a no brainer for someone like myself.