Start The Emissary by taking the seal from the carriage near The Other Side Lantern in Valley of Ashes. You cannot finish the poison route during this first visit: continue the story until you obtain Shield at the University, then return to investigate the camp. A nearby chest contains Perfect Copy Amber.
Take the Emissary’s Seal
From The Other Side, follow the left passage toward the poison-covered camp. Search the luxurious carriage and collect the Emissary’s Seal from its desk. This starts the investigation into the missing envoy.
You can clear a nearby obstruction before leaving. Reach the elevated route near the ladder shortcut, find an angle on the glowing growth, and shoot it to open the passage. Clearing the growth does not make the poison safe.
Return with Shield
Continue the main story until the University grants Shield Transmutation. Return to The Other Side, equip Shield and activate it while crossing the contaminated route beside the carriage. Reach clear ground before stopping its protection.
The Emissary’s Letter, found along the route after Slaughtermill, provides an earlier clue about the envoy. It does not replace the later investigation beyond the poison.
Investigate the soldiers and red cloth
Beyond the toxic area, read the note beside the fallen soldier. Continue until you find the soldier surrounded by bodies and corruption. Inspecting him triggers an encounter with a stronger Risen Soldier, so finish the fight before searching farther along the trail.
Find the red fabric near the gate beyond the encounter and examine it to complete The Emissary. The fabric is the final quest clue; simply crossing the poison does not finish the investigation.
Open the nearby Perfect Copy chest
Before leaving, check the nearby tent and chest. Perfect Copy is a separate pickup along the final route and can be overlooked if you leave as soon as the quest completes. This chest is the destination for the Amber; the seal and letters are investigation items.