You can start this quest by reading the "Contract: Monster in the Cemetery" notice posted on the Lindenvale Notice Board or by talking to the Gravedigger directly.
Geralt took a contract the gravedigger of Lindenvale had hung on the village's notice board. It seemed the village cemetery was full of monsters who seized children as their victims.
Lindenvale Notice Board
Velen / Novigrad
Gravedigger
Velen / Novigrad
Cemetery
Velen / Novigrad
Grave Hag Lair
Velen / Novigrad
Mourntart
Velen / Novigrad
Talk to the gravedigger. He'll tell you about the problems at the cemetery and the disappearance of a little boy from the village. You'll be able to bargain with him about your payment - 240+ crowns.
Go to the Cemetery and kill the rabid dogs (5) you encounter there.
Examine the recently dug-up grave.
Follow the scent of cadaverine. It will take you to the old hut where the Grave Hag Lair is located.
Enter the hut and examine the clues you find there: tools, cauldron and skulls. When you do this, Geralt will conclude that he is dealing with a grave hag and that if he takes the skulls, she will have to go to the cemetery to get new ones. Take the Grave hag skull.
Geralt investigated the matter and discovered a grave hag hunted nearby. With the investigation part of his task complete, it was on to the luring-out-and-slicing-to-ribbons phase.
Go back to the cemetery and put the skulls on the pedestal inside the chapel.
Geralt automatically waits until night comes and Mourntart appears. Once she's dead, collect the Grave hag trophy (5% Bonus experience from humans and nonhumans).
Go back to the gravedigger, collect your reward and inform him about the fate of the missing boy.
After ridding the village of the monster, Geralt collected a trophy from its body and went to the gravedigger for the promised bounty.