Respect Thy Brother
You'll get the hook for this mission during the Walls of the Ruler story mission, but the mission proper begins inside the fortress of the same name, when you find the body of Hidden One on the 'main stage' in the courtyard.
Your goal is to find a burying place, but but don't worry about that yet- first just carry him out of the fort.
Once you exit the hostile area, you'll get a marker by the Necropolis- the same one as in the Howls of the Dead side quest. Put your fallen comrade on a horse and carry him there.
Getting close to the marker automatically triggers a cutscene which completes the mission.
Note that there is a small box of carbon crystals in this area as well, on your right as you approach the gravesite.