In folklore, Will-o’-the-wisp are atmospheric ghost lights often seen near bogs or swamps. Nymphs can mean a lot of different things, but most mythologies depict them as women nature deities. This painting depicting both fictional entities is by Georg Janny.
Besides for the Nymphs and Will-o’-the-Wisp – I notice a rather creepy tree monster!

Image is from Wikimedia Commons and is under Public Domain License. Information is from Wikipedia(CC BY-SA 3.0).
With Love, TeeTee Ella