Mission 7 - The Flooded District
To spare Daud's life, blink until you are close enough to pickpocket his pouch from him, and steal the key, then immediately blink away out of the window and you can exit the area, as opposed to killing him and grabbing the key.
if he catches you trying to steal it, just wait till he releases you and blink away, he won't pursue.
This achievement will pop once you leave the area via the tunnels you used the key to reach.