Go to Tarantula Lake and look for the character named Evelyn (she appears around the lake in different places). She will only speak to you during the night. Say Wisp Ring and she will ask you for a packet of willow bark you get from the Save the Little Tree quest and an iron ring (which you can easily find on Arakas).
Once the two objects are reunited, return to see her and tell her again Feu-follet ring then Yes and she will give you the Feu-follet ring.
Note: If you have several Willow Bark Bags or Iron Rings put them all on the ground, otherwise Evelyn will not want to give you the ring.
Gain:
You can do this quest at will.