You will have to make sure you have The War-Chief's Trials quest active in order for you to proceed with the quest.
Follow the quest marker on your user interface and you will come across a Grazer site. Sneak through it and get to the objective zone. Use your Focus to scan the area for clues.
Sona's Last Known Location
Follow the highlighted path and be careful for there will be many machines in that area. The location will be on top of a stony hill top.
A Tallneck will also be in the area. You should take the time to climb up the Tallneck to gain experience and unlock a portion of the map's fog of war.
Once you reach Sona's last known location scan the area with the Focus in search for clues. Follow the clues to a dead Sawtooth and a cut-scene will initiate.
Kill the Cultists and Corrupted Machines
You will now have to kill the cultists and the Corrupted Machines. It would be best to kill a few of them by shooting the blaze canisters scattered around the dig site with fire arrows and then snipe some of the cultists from afar instead of going straight down in to the dig site to fight them.
Investigate the Dig Site
Once you've defeated all the enemies, use your Focus to scan around the dig site for any clues.