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This achievement can be earned at any time during your playthrough, specifically on the lethal High Chaos run.

You will be introduced to Drop Assassinations fairly early on in the game during Mission 1, A Long Day in Dunwall. While perched on a ledge on any kind (rooftop, railing, etc.), you can eliminate enemies who are below you by jumping off the ledge and falling on top of them. Right before you make contact with the enemy, a quick prompt will flash on the screen: [ Register or Signin to view external links. ] to knock out or [ Register or Signin to view external links. ] to kill.

There are several opportunities across all nine missions in the game to get this done, but do keep in mind that you must kill the enemies rather than knocking them out. The achievement will unlock once you have performed ten successful Drop Assassinations.