Forefather's Eve - Velen
Location: Outside Keira's Hut or Crow's Perch
Quest Giver: Pellar
Other Requirements: Complete A Towerful of Mice
The Pellar will contact you sometime after completing the "A Towerful of Mice" quest from Keira. Where he contacts you depends on whether or not you've completed the Family Matters quest. For this quest you will need to head back to Fyre Island and meet the Pellar at the quest location at 10pm or later.
For this quest you'll have to protect the Pellar as he performs a ritual then mid way through make a choice that will alter the outcome of this quest. A group of Witch Hunters will show up and you'll either have to protect the Pellar or let them interrupt the whole ritual. Letting them interrupt will fail the quest.
Once the ritual is finished you will be tasked with going out to the swamps in Crookback Bog and burning the Pellar's father's corpse. It's easy to find, right in the middle of the quest marker (shown in the screenshot below) and guarded by a few Rotfiends. All you have to do is use Igni on it to torch it, then you can return to the Pellar.
Completion of this quest will unlock the Pellar as a merchant. He is an Alchemist who doesn't have that great of a stock, the rest of this quest - nor the choices you make during it with the Witch Hunters - has any bearing on any future quests.