Sadako vs Kayako (The Ring vs The Grudge)

(The trailer is in Japanese)

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Supposedly started out as an April Fools joke, but because fan response was huge it was decided to make it a real movie.

Interestingly, the director's first movie Ju-Rei: The Uncanny (2004) is basically a rip off of The Ring and The Grudge.

The director also made Noroi: The Curse (2005), and excellent creepy found footage film.