Contract: The Merry Widow - Velen
Quest Giver: Lindenvale Notice Board or Discovering Location
Other Requirements: N/A
If you accepted this quest from the Lindenvale Notice Board then the first thing you will have to do is speak with the gravedigger in town to update the quest. After speaking to the gravedigger you'll be sent to the east to investigate the cemetery, it'll be marked on both your map and compass.
When you arrive you'll find some Ghouls to take care of, after which you'll have to use your Witcher Senses to investigate the graves on either side of the building and in front. The grave on the right hand side of the building (if you're facing the entrance) advances the quest forward and gives you a scent which you will need to follow to the next quest location.
The scent leads you to an abandoned house along the road south of Lindenvale. Here you'll have to investigate everything in the room before your quest updates and tells you to pick up the three skulls in the corner. Grab them and then return to the mausoleum in the center of the graveyard.
After placing the skulls on the pedestal inside of the mausoleum there will be a short cutscene which also throws us into a time portal and makes it night time. Mourntart, the name of our grave hag will come out and attack you at this point. To save you some time reading her bestiary, I will include what she is most vulnerable to below.
Grave Hag Vulnerabilities: Black Blood, Necrophage Oil, Yrden, Quen
Once you defeat the grave hag take the trophy back to the gravedigger in Lindenvale for your reward.