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.
This is legit such an underrated gem of an anime, at least underrated in the west. It’s so good.
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.