In the area after you first start the game, just after you see the Knights 
Templar, enter the room containing the Ark Of The Covenant. Move towards the 
ladder in front, and jump to the left of it, as if to grab onto the ledge on the 
wall. You should fly through the wall and land behind it on a square outline. 
Walk towards the middle of this square, and you will fall through, dying shortly 
afterwards. After you die, you should get the "Eagle's Will" achievement.