is the storyline of the assassins creed series so far based on true story?

[Devil]

i dont really understand the main storyline to both games (AC1 and AC2) and i have just unlocked all over subject 16's crap. ive seen the video and looked up some explanations and it all seems to be very realistic, as if "the piece of eden" is actually a real life object. im only young so i dont understand this sort of stuff. can any explain me the storyline in a few sentences and answer my main question. thank you