**Note that this was my personal way to unlock this achievement so I cannot quarentee that this will unlock to you too in same way ! **
There might be some grammar errors so I apologize in advance.
This is a witcher contract that can be obtained from Odhen of Rannvaig named NPC who is located in a village Rannvaig in the Skellige Isles. You will get this contract started also by accepting a notice board notice of this contract. *Contract name is MISSING SON*
Suggested lvl for this contract is lvl 29 but I managed to complete this as I was only lvl 25. I did have some good gear though but I am sure that at this part of the game, you will also have some very good gear etc. The name of this contract is *Missing Son*
The monster of this contract is a Fiend called Morvudd. So it has a Fiend fighting style that you have to keep in mind: ramming attacks, large sweeps and deadly blows. I personally used only heavy armor gear, decent weapons, Igni sign and Swallow potion + food. Morvudd did some heavy damage to me with it's every blow so I suggest to dodge more often than tanking damage.
Bestiary says that Morvudd is vulnerable against these: Sanum and Devil's Puffball bombs and Relict oil. I suggest just in case to craft these and use them for getting that extra edge for the fight.
Here is the walkthough how I personally completed this contract. You can complete this in a different way I believe:
1. I accepted this contract from the notice board that was located in a Rannvaig village.
2. Then in a heed of mind, I didn't talk to Odhen at all but instead randomly left from the village and head south towards some huge castle ruins that are located just north of village called Boxholm.
3. There, in these particular ruins I encountered Morvudd and the monster life bar suddenly appeared in my tv screen. The fight had started.
4. When the monster had around 25% health left, it's center eye started to glow with a red light around it and suddenly I was hypnotized for 20 seconds. Looks likely this is a part of the contract because the monster fleed from the battle back to it's lair.
5. I used witcher senses as commanded, to track down this monster towards it's lair. The lair was located in village of Boxholm.
6. There a second fight began againts this beast but this time it did have a full health in start. I eventually killed the beast, used witcher sense in it's lair, located a body without a torso and returned to Odhen to tell that he's sons have died.
7. After the conversation my achievement unlocked. Hopefully yours will too !