You’ll pick this quest up from Varl as part of the main storyline. Your first task is simply just to travel to the massacre site. When you get there, use your Focus to investigate the site. As you do a survivor will turn up. Talk to him, then head for Sona’s last known location.
When you get there, again, investigate the area with your Focus and then, once you’ve done so, highlight the tracks and follow them across the tops of the cliffs. Be aware that there will be Corrupted Machines along this path, so either sneak by them or take them out (with care), until you find the Sawtooth body and the cutscene starts. When it’s over, follow Sona to the edge of the Cultist camp. There will be another cutscene then you’ll be tasked with clearing out the camp. There are a number of human enemies, both ranged and melee, as well as a number of Corrupted Watchers patrolling. If you have the Silent Strike skill now’s the time to start using it.
If that’s not your style, there are Blaze canisters scattered around the camp that you can shoot with fire arrows to make big booms. Whatever way you want to go about it, it’s likely you’ll end up in a fight with the bad guys eventually, but don’t worry, you’ll have backup from the Nora Warparty, who’ll sweep in as soon as the alarm is raised. Kill all the bad guys then talk to Sona. When that’s done, investigate the Dig Site, listen to the audio file you find, then report back to Nora and Varl to finish this quest.