How to Acquire
To unlock this achievement, Corvo Attano or Emily Kaldwin must eliminate the Crown Killer in any fashion. This can be done in two ways. The first, and probably easiest, is to kill the Crown Killer. This can easily be done by triggering Doctor Alexandria Hypatia's transformation in such a way that the Crown Killer is not aware of the protagonist's location, then stealthily killing her.
The more complicated way is to talk to Doctor Bartholomeus Vasco, or loot his body for the note, and follow the instructions to create the counter-serum, curing Hypatia of the Crown Killer. If this method is followed, the achievement Counter-Serum can later be unlocked.