Ghost (achievement)

Ghost is an achievement/trophy in Deus Ex: Human Revolution. It is earned by getting through a hostile area without raising any alarms.

Overview
To get this achievement you need to get through a hostile occupied area without setting off any alarms. You are allowed to kill/knockout guards as long as they don't turn hostile towards you.

Strategy
This is actually one of the easiest achievements to get, despite its intimidating description.
 * The quickest way to get this achievement is on the very first mission (Back in the Saddle) you receive after being augmented. After you arrive at Milwaukee Junction, head down to the ground level (Pritchard will contact you and suggest attempting a stealthy entrance). Around the corner will be a guard with his back to you and talking on the phone. Use a takedown to take him out (lethal or non-lethal, as you prefer). Along the wall on the right is a crate sitting in front of a forklift; take the crate and place it against the steel container to climb up onto the container. Keep climbing up until you reach the roof. You'll notice the path ahead is blocked by exposed electrical cables, but there is a vent hidden behind a small crate to the right; move the crate and crawl through the vent. When you emerge, use the switchbox in front of you to turn off the electricity, then continue along the roof until you find another vent leading inside the building. You'll gain the achievement as soon as you go inside.
 * In the first mission (when Adam is augmented) choose in the helicopter for a non-lethal and then the tranquilizer rifle. As the helicopter lands and you get down on the floor the first guard is just around the corner. Take him down with a silent takedown. A bit further are 3 more guards. Wait a few seconds to analyse their patrols and then start taking them out. First the one at the far end (next to the door). The second one should be the one patroling in the middle, shoot him also with the rifle. Then crouch sneak to the one who is standing the closest to you and take him out with a silent takedown. Then enter your objective building. The achievement pops up.

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