Ok, this doesn't make any sense. "unholy terror" is usually used to describe a kid who acts out so much people think he is possessed (that's a rough interpretation). So, is the intended meaning 1) "something which causes trouble all over the world" or actually 2) "terror (fear) from the devil spread throughout the world?"
1) 世界中のとんでもない悪がき - "horrible kid throughout the world" kind of doesn't make sense. how can a bad kid be worldwide
2) 世界中の邪悪な恐怖 - "evil fear throughout the world" makes a little bit more sense.
Tell your client that double entendres do not work the same in two different languages. He will have to choose a concrete meaning and go with that.