Razor Rain Achievement in Dishonored
You killed 5 characters with Drop Assassination
How to unlock Razor Rain
When above an enemy, drop down and press when prompted to stab/kill them. This is introduced to you in the game while trying to escape the prison. You cannot reload checkpoints to get these, they must be on 5 different enemies. You will have many opportunities to do this, but be sure to remember that this will void any no-kill achievements you are going for.