This quest comes after Field of the Fallen. Head for Meridian, then go to the Sun Palace to talk to Erend. When you get there you’ll meet Blameless Marand, who’ll take you to Avad, the Sun King. Talk to him and Erend then head for Pitchcliff. When you get there, follow your waypoint to the market to trigger a cutscene. When you’re back in control, scan with your Focus to find the tracks then highlight and follow them.
Follow them until you find the agent’s body, investigate it to trigger another cutscene. When you’re back in control, head out of Pitchcliff and make your way to Dervahl’s camp. Watch for various aggressive versions of Machine wildlife along the way. Once you get to the camp, there will be a short cutscene, then you’ll have to get inside the camp. Usefully there’s a Ravager and Longleg chained up outside. Get close and destroy their restraints (and also override them if you want) and they’ll cause all kinds of chaos in the camp.
The camp has two layers, so clear out the lower layer, then loot and heal as needed, then head upstairs. If you managed to kill all of Dervahl’s men in the lower layer, there will just be a restrained Longleg and Ravager down here. Free and override them both as there’s a mini-boss coming up.
Follow your waypoint to the door and a cutscene will trigger. When it’s done, you’ll have an Elite with a firestriker to deal with, which is where the Longleg and Ravager come in handy. Kill the elite however you can and when he’s down, loot his corpse to trigger another cutscene.
Head downstairs to trigger another cutscene where you find Ersa’s fate. Back in control, investigate the workshop, including the sound weapon, the machinery and the recording box. When you’ve done that, talk to Erend again to complete this quest.