Was there an actual Assassin order? If so where were they all located? Why and who did they assassinate? Did they have their own ranking system? Who was their Leader?
The assassins in "assassin's creed" are somehow based on the the Nizari Ismailis Ḥashshāshīn order that arose in the late 11th in Syria and Persia and opposed the authority of the Seljuk Turks.
They were pretty much destroyed by the Mongol Empire, but their legacy was preserved in Europe thanks to the writings of Marco Polo, who depicts them as trained and skilled killers used for systematic elimination of opposing faction leaders. This term was then changed over time to "assassin" and is pretty much exactly what the word means today.
How they have any relation to the assassin's in "Assassin's Creed", I have no idea.
The leader of the Ḥashshāshīn's was a missionary called Hassan-i Sabbah, who founded the order in a mountain fortress called Alamut.