"Forefathers' Eve" is a secondary quest in The Witcher 3: Wild Hunt that focuses on the Pellar, a character who plays a role in the Bloody Baron's part of Velen's main quest. While the Pellar can help Geralt in several ways with the Baron, in "Forefathers' Eve" it's the Pellar who needs help.
Firstly, this quest is only available after Geralt completes The Witcher 3 side quest "A Towerful of Mice." Once it's done, the way players get this task depends on how far into the Baron's quest they are. If Geralt has already gotten the Pellar's help during "Family Matters," he'll speak to Geralt near the blacksmith in Crow's Perch. If Geralt hasn't gone looking for the Pellar yet, the mystic will speak to him in front of Keira's hut. If Geralt is currently looking for the Pellar, then players will have to complete the next phase of "Family Matters" before the Pellar will ask for help.
Fyke Isle
When the Pellar contacts Geralt, he asks for protection on Forefathers' Eve, a day when locals send troubled spirits to their final rest. Some of these spirits are violent, which is why he needs a witcher. The rite takes place on Fyke Isle, which is the same island "A Towerful of Mice" takes place on. This is why that quest has to be over first. Luckily, this also means players will have a signpost on the island and so they can get to the ceremony grounds quickly.
Once Forefathers' Eve begins (players must meditate until midnight if it's not), a group of drowners led by a water hag attacks. After Geralt defeats them, a spirit from the Pellar's past will appear. However, before he can do anything a group of witch hunters will interrupt the ceremony. Geralt can choose to stop them or stay out of it, but if he stands back the quest will end here. If he intervenes, Geralt will have to kill the witch hunters and their deaths will taint the rite. This will summon several wraiths that Geralt will also have to fight.
The wraiths end the rite early, but because Geralt helped the quest isn't over. Instead, Geralt volunteers to help the Pellar with the ghost from his past, and with the ceremony spoiled the only way to do this is to find and burn the ghost's former body.
The Swamp
The Pellar will tell Geralt that he left the body in the swamp, which is the same swamp as the main quest "Ladies of the Wood." This secondary quest has nothing to do with "Ladies," but players may end up starting that quest accidentally if they wander too far.
Once Geralt reaches the area indicated by the quest, he'll come across a group of rotfiends. He'll also find a man who can give him the contract "Swamp Thing" if players haven't already started it. Close to the rotfiends is a blackened body that glows red when using Witcher Senses, and all players have to do is hit it with Igni. The last step of this quest is to find the Pellar, who's back in his hut outside Blackbough. Geralt can ask him about the spirit, but either way the Pellar will give Geralt 50 crowns and a permanent discount on all his wares.
The Witcher 3 is available now on PC, PS4, PS5, Switch, Xbox One, and Xbox Series X/S.