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The earliest you can work on this achievement is during Mission 2, Edge of the World. Take note that this can only be done with Emily, as Corvo does not have the Domino ability.

Upon reaching the Karnaca docks, you will be tasked with getting to Addemire Station. Before you set out, equip the Heart and make your way to the nearby Rune on the docks, located inside a dead whale's mouth. Collect the Rune and now travel to the Black Market down the road, where you can purchase a second Rune for a small sum of money.

At this time, open the character menu by pressing [ Register or Signin to view external links. ] and scroll over to the "Powers" tab. Using your four Runes, unlock the first Domino skill and then close out of the menu. Your next task will be to continue moving though the streets of the city towards Addemire Station. Eventually you will reach a Wall of Light. There are two guards here, one of which is pushing a civilian towards the electric barrier. Quickly equip Domino, target your reticle at the left-most guard, tap [ Register or Signin to view external links. ] to cast the ability, and then link him to the guard on his right.

After the civilian has been vaporized, rush up to the guard on the right, hold [ Register or Signin to view external links. ] to grab him, and as the other guard becomes alerted and swings his sword to attack you, he will kill his ally and as a result kill himself, thus unlocking the achievement.