How to Complete I Want to Forget
This quest becomes available after you have completed I've Been Had, Kupo!.
First, go to the Witch's Hut in Tramdine Fens. She will tell you about Zombie Powder.
Afterward, go to the quest location next to the Witch's Hut. Ezel will tell you about the Lethean Draught.
If you kill only the Dreamhares, you will make Lethean Draught, allowing Glefein to forget about the unrequited love. If you kill only the Zombies, you will make Zombie Powder, turning Glefein into a Zombie. If you kill all but one Zombie and one Dreamhare, then kill both at the same time with a spell, then you will make Zombie Powder.
It is your choice whether to kill the Zombies or the Dreamhares. You will successfully complete the quest either way. The outcome does not affect the rest of the game, and you never meet the petitioner. This quest is simply meant to raise ethical questions for you to ponder.