Can anyone provide some spoiler-free clues as to where to start looking for the quests that start this?
It is located in forest, covered in fog, which is north of Hyrule castle. Enter the forest from the south-east and you will see various torches. Walk in the direction that the embers are blowing. At a certain point, you will need to light your own torch and keep stopping, to see if the embers have started blowing in a different direction.