Looking for the good stories where Druids take part in the fight against Voldemort, on their own or alongside the canon characters.

 

It's not exactly what you're looking for, but Prince of a Dark Kingdom has Voldemort being pagan; his immortality granted by Gaia to restore magic to what it should be. The later books (4+ if I remember correctly, have a lot of paganism practices and myths scatter through it; Harry becoming a pagan being an important theme of the story.