Natsume Yujin-cho

Natsume is attacked by a youkai while retrieving a ball at school. In his haste to run away, he ends up breaking a youkai's seal, just like how he met Nyanko-sensei. Later that night, Natsume discovers a black lookalike of Nyanko-sensei, only to have him steal the Book Of Friends! As Natsume and Nyanko-sensei search for the black cat, they stumble across a gathering of youkai. The youkai plan to attack the humans for sealing their lord - the black cat Natsume's looking for. With Nyanko-sensei's help, Natsume gives back the lord's name and catches a glimpse of his fond and bittersweet memories of living among humans. (Source: Wikipedia)