Missing Son is a Side Quest that can be started at the notice board in Rannvaig, a village on the main island of Skellige. 

The quest is fairly straight forward, yet no easy task at all. You must fight one of the strongest monsters in the game, called Morvudd. It will kill you in 1-2 hits and has quite a bit of health. You must also defeat it twice in a row, as it will run away the first time you fight it.

The basic strategy for this fight is to dodge Morvudd's attacks to the left at all times, then use one single strong attack and repeat. This way you can keep going around the enemy in circles and if you get the timing down the boss will hardly ever hit you while you deal some devastating blows. Also use Relic Oil on your silver sword for bonus damage and bring some Samum bombs.

Keep your Quen Sign active at all times, otherwise you may get instant-killed by the enemy. Once you've defeated Morvudd for the second time you must investigate its lair and return to the quest giver. This will complete the Missing Son quest and unlock the Fiend or Foe? trophy or achievement.