Above is a walkthrough on how to obtain the breeding task: Widow's Juicy Elf from the Widow in Amorous Hallows cave, how to pick up the Sylvan Treehouse, capture elves, and complete the task.
For reference, Elves like Demon or Seraphim Semen, which you can trade/buy from Camilla in Hedon Township! You can give the semen to the elf you want to capture!