Once the main character of the original AC, it was a somewhat risky but awesome idea to frame Altair's future exploits as the backdrop of sequel-hero Ezio's newer adventures. Come the point we'd finally get to control the eagle-hooded legend again in Revelations, it marked the end of both Altair and Ezio's respective storylines, simultaneously.
Now a very old and dying man, Altair is seen locking himself away with the one Piece of Eden (mystical artefacts of great power), knowing that eventually, Ezio would come to find him. The two characters are hundreds of years apart, and the way the camera swoops slowly around the dying Altair as we then come back around on Ezio finding his now-skeletal remains, is just brilliant.
The fact that our formerly-humble servant of the Assassins actually helped guide the order and become revered as a legend in between the games was a driving force to uncover everything he'd been up to. And for that to then intertwine so poetically with Ezio's own tale, uniting the characters we'd grown to love and anchoring them to a point in time, was nothing short of incredible.