Natsume is jumped by a group of Youkai trying to steal the Book of Friends for their master. Once captured, Natsume discovers that an old problem is back in a big way.
They're probably what people 
call "youkai", Japanese monsters.
Probablemente son las cosas 
que llamarían youkai.
Aquellos seres a los que llaman youkai.
Vejo o que costumam chamar de youkai.