"Dahl Ultimatum" and "Incapacitate Dahl" are related objectives that are also part of a larger series of intertwining story objectives that take place as Prey's conclusion nears. Though you're free to complete many of these objectives at your leisure, there's a more efficient way. We've corralled them into our "The Keys to the Kingdom" guide, which encompasses several related objectives. This guide focuses "Dahl Ultimatum" and "Incapacitate Dahl."
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Dahl Ultimatum and/or Incapacitate Dahl
These two objectives are closely tied — in fact, if you complete "Incapacitate Dahl" fast enough, he’ll never get around to starting "Dahl Ultimatum," so we’ll start there.
Assuming you completed "Doctor Igwe" and saved him during "Shipping and Receiving," Dr. Igwe will call you after Dahl’s arrival with a plan. You just have to incapacitate Dahl without killing him. But you’ll have to find him first.
He’s down in Life Support in the Oxygen Flow Control Room, which is on the right side of the Atmosphere Control Room. (You’ll be on the other side of Atmosphere Control during "Missing Engineer" optional objective.) With your tracking bracelet deactivated, he won’t see you coming. Just draw your Disruptor Stun Gun and drop him. This will let you completely skip over "Dahl Ultimatum."
If you don’t take out (incapacitate, above) Dahl before you head toward the Shuttle Bay, he’ll threaten all of the survivors down in the Cargo Bay in an effort to dissuade you. He’ll use the controls down in Atmosphere Control to pump the oxygen out of their sanctuary.
From here, the process is the same as above. Spoiler: He’s in Life Support in the Oxygen Flow Control Room which is on the right side of the Atmosphere Control Room. This time, though, when you incapacitate him (to whatever degree of permanence you can morally justify), you’ll also have to repair the Talos 1 Oxygen Control panel on the right side of the room.
Incapacitate Dahl — Activate the Neuromod removal process
If Dahl survives his encounter with you during his ultimatum or when you incapacitating him, Dr. Igwe will retrieve him and take him to Neuromod Division. You’ll get another call from Dr. Igwe asking you to meet him in the Neuromod Insertion Room.
The Neuromod Insertion Room is on the second floor of the Neuromod Division. You may have been up here before to explore the Volunteer Quarters and the Simulation Debriefing Room. When you meet up with Dr. Igwe and the unconscious Dahl, everything is prepped. All that’s left if for you to push the button(s).
Go to the computer next to the chamber. You’ll have to hit a series of buttons here. On the Surgical Procedures menu, press Engage Surgical Harness. Then switch to Neural Scan, and hit Preform Neural Scan. Now you can switch back to Surgical Procedures to choose Perform Neurotomy. (You can play and stop the debriefing as much as you want, but it doesn’t affect anything.)
After you and Igwe have handled Dahl, the "Incapacitate Dahl" objective will go on hold until you complete "The Repo Man" (which we’ve been ignoring so far in this guide). So let’s take care of that now.