This page in our guide to GreedFall contains a walkthrough for Searching for Constantin main quest.
After completing other main missions, go to Constantin's mansion. You find out that Constantin has disappeared. Go to Magasvar and find Constatin's camp. You now have to follow the trails of corpses and traces of fights. Find more evidence and clues, and then move on to a new part of the map.
You end up at the Alliance Outpost Camp. Go inside and speak with the soldiers - they will tell you what has happened. An extremely important witness is burned and in serious condition. The medic will ask you to create a potion against burns - you can make it at a workbench.
Give the medicine to the woman and wait 24 hours (as always, go to the campfire to speed up the time). Then, speak with the wounded and talk to Captain Idris - this will end the mission. You get 2,000 XP. You will also immediately start The Dead Man's Voice main quest.