Natsume's Book of Friends

Tanuma's father leaves on a trip and a strange youkai invades his house, spreading miasma that makes all the nearby youkai suffer. Natsume tracks it down and discovers that the noxious youkai was once a resident in an all-youkai village. A portal had opened for a short time during a human ceremony that he had taken his sister to watch. However, they became trapped in the human world, and his sister died. After returning to the old temple where the ceremonies were held, the portal opens and he returns home.