I've recently discovered this hidden gem. This is a much darker Anime then I expected. It is created by CLAMP and is seinen (for adults 18-30). It's got a little bit of everything as most good Animes do. What's unique about this Anime is how they blend very interesting mystical themes with modern life and it's all done with awesome animation. It's quite nice because you get some interesting insights into Japanese superstitions. Honestly it can be a little chilling sometimes. The Japanese voice cast is great and the music is very fitting. So far I've seen 19 episodes (Animesuki) and I'm looking forward to more of this unique experience.
Review of XXXHOLiC
