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. Approach the ladder in front and jump to the left of it, as if to grab onto the ledge on the wall. You will 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 will unlock the "Eagle's Will" achievement.