This is story related and cannot be missed. After unlocking “The Birth of an Assassin”, when you again take control of Desmond, you must follow Lucy and escape Abstergo. Along the way, you will make your way through a maze of Animi. Just follow Lucy through it. At the end, Lucy will unsuccessfully punch a code in to open an elevator door. Press a button to use your Eagle Vision to figure out the code and the door will open. Upon exiting the elevator, you will encounter multiple guards. After taking them out, Lucy will tell you to get in the trunk of a car. When the lid closes, the achievement will unlock.