Source (in Japanese). This is technically not a visual to commemorate the anime, but rather it is to promote a musical concert celebrating the 15th anniversary of the anime’s premiere. There is a seventh season of the show that has been announced, but as of yet, no release date has been provided.
I originally came across this anime when I was looking for shows that were similar to Mushishi. It ended up not really scratching quite the same itch, but it is very good. I agree that it is really underrated in the West as I hadn’t really heard much about it before starting it.