Prey

Im suprised that there is no guide here. But it makes some sense since this achievement is tied to other achievements and decisions the player has to make in their game. Even though a guide may not be necessary for this achievement, I’ll try to contribute. I will be more than happy to write a guide for this awesome game, even if it is a single achievement. Im also adding two new ways to knock out Dahl, most people are not aware of them. Credits go to Heraizen cause his guide to kill all the humans is tied to this achievement.

First of all I highly suggest to go for it in your evil/kill all humans playthrough. You need to make sure that you have done the below steps:

1. Kill every human so far but save Dr. Igwe (Adrift)
2. Make sure Dr. Idwe gets in Morgan’s office in Talos 1 Lobby (dont kill him in Cargo Bay)
3. Igwe's “Incapacitate Dahl” will start but you dont have to meet him in Morgan's office

Paths for the achievement (path 2 doesnt fit in an evil playthough but just in case..):

1. If you have killed all the humans you encountered by yourself, Dahl will consider you his ally. His quest “Taking initiative” will start. Follow it, kill the 4 persons he wants dead and meet him in the Shuttle Bay (he betrays you here). Knock him out with your stun gun or the Electrostatic Burst typhoon ability.

2. If you are not killing humans you should really start but if you spared Elazar’s team (Cargo Bay), the following will happen:
1) If you go after Dahl, he will threaten Elazar’s team in Life Support. Go after him and knock him out with your stun gun. (This is a solution for people that don’t have the stun gun, by using the decontamination system in the opposite room where he is: You have to remove an object from the fans. Firstly, turn them off for some time by the console and then go to the decontamination room and initiate the procedure. Dahl will be knocked out cause of oxygen loss).
2) If you destroy his operator officer first, he will threaten Alex. Knock him out. Here I suggest not to the trigger the last quest yet.

(Now you need to kill any survivors that you missed and then) go to the Neuromod Division where Dr. Igwe is waiting. Perform the surgery (This Never Happened) and kill Igwe but not Dahl. Here you have to make sure that you have killed 41 people (or 40 with Alex still alive, you can kill him later just before the ending too). Credits here go to Alice2183 cause I watched her videos to make sure about the order of this moment's events and the body count

Time-saving note: You can also perform the surgery after triggering the last quest but I do suggest killing humans before doing so. If you kill Alex in the last quest and all humans except Dahl and Igwe are dead (40 in total), you are good to go.

Obviously, you need to Dahl is waiting for you in the Shuttle Bay. Enter his shuttle and get the hell out of there. Achievement unlocked.