Contract: The Merry Widow is a side quest in Velen.

Head to the notice board in Lindenvale and pick up the Contract: Monster in the Cemetery. Immediately head to the marker and talk to the grave digger. He will tell you that someone has been digging up graves in the cemetery and a boy has gone missing. You can haggle for a price, I settled at 265.

After you talked to the grave digger head to the cemetery. As you get there you will notice four or five level 5 wild dogs. Quickly kill them and then use your witcher senses to examine a grave. Then head to the right of the building and examine that grave. This particular grave will have an odor follow it to a nearby abandoned house. Once inside examine the tools, pot, and skull. Pickup the skull and then head back to the cemetery.

Once in the cemetery go into the building and place the skull on the altar. After you do that a grave hag will appear. Use your Yrden to slow the hag down, while you use your thunderbolt potion to add extra damage to your attacks. Always dodge the hag as it approaches to avoid getting hit by its long range melee attacks. Also watch out for it tongue attack.

After you killed the Hag loot it and then head back to the grave digger to get your reward.