Mercy is the Mark

Mercy is the Mark is a hidden achievement/trophy in Dishonored.

How to Acquire
The achievement can be earned by choosing the non-lethal way of neutralizing Daud during the mission The Flooded District.

Trivia
This might be a reference to a line of the Sci-fi series: Firefly which is spoken by Captain Malcolm Ryenolds after winning a duel and choosing not to execute his opponent.

Gnade vor Recht Милосердие Il Marchio Della Pietà Clemencia es la señal