Forefather's Eve is a Side Quest given by the Pellar in Velen once you have completed the Tower Full of Mice quest.

Forefather's Eve Walkthrough

After accepting the quest, meet the Pellar. Meditate until it's midnight and then talk to him. The ritual will begin and Drowners and a Water Hag will come out of the water and will attack. If you're having difficulties defeating them, remember to apply Necrophage Oil and equip Igni.

The Pellar will summon Ambros, his dead father in the flames. When three Witch Hunters arrive, you have a choice to make. Defend the Pellar and attempt to tell them to leave him alone, or give up and end the quest.

If you stand up for the Pellar, you will have to fight the Hunters. Slow them down and take them out one by one. This battle will unleash three Wraiths. Defeat them, in turn, to continue to the quest. 

The Pellar will ask you to find his father's body. Go to the west and follow your map marker to the swamps. Using your Witcher Senses, you will discover the body. Destroy it with Igni to complete the quest.