Shadow Achievement in Dishonored
You completed all missions after the prologue without alerting anyone
How to unlock Shadow
In order to know when an enemy is alert, watch the lightning bolts over their head. Once they are all filled in with white, they will turn red and the enemy will start attacking you. If that happens, reload the last save you have. Always pay close to attention to the detection markers (the lightning bolts) and as soon as you see even a bit of white, hide until the enemy moves on.