Storyline achievement. (Kinda, You're unable to unlock it until after the main storyline.)
After finishing the game's last mission by kiling Al Mualim, the mentor of Altair, you are brought back to the lab. Use eagle vision (Y) in the bedroom to see the images that the previous subject wrote in blood.
There are several areas with images but the important one is on the wall above your bed. After seeing this area it should unlock after the credit sequence.