Note: Getting the quest by examining the body means you can't start the quest by talking to the father Oghden and getting all the details of the quest.
Quest Description Edit
Objective 1 Edit
To finish the quest you gotta defeat a special fiend. To kill it you gotta either outlevel it in equipment and stats or by timing your dodge and attack with his attacks. What worked for me was using relic oil on silver sword, thunderbolt potion for damage buff, swallow for healing and quen to negate one of his attacks. I would approach the fiend, wait for it to start its attack animation and then dodge to its side, attack with fast attack and wait for its attack animation to start then dodge again. Of course used quen before I approached. Sometimes he would start his attack a bit later and it would throw your timing off. Thats what the quen is for. Don't use jump away to dodge his attack because it sends you further outside of your attack range and depletes your stamina, which you need for casting quen.
Additional Info Edit
Closing Quest Description Edit