When a white man uses tribal magic to kill people with their own shadows an Aboriginal youth is sent to stop him from destroying reality.

Try it the other way around. Start with the protagonist, spell out his goal and his conflict:

"A young Aborigine must stop a white man using tribal magic to murder people with their own shadows."

Stopping a killer is enough of a necessity that you don't need to mention the end of reality in the logline. In a synopsis, sure, because you can spend a couple sentences explaining what that means. But you don't want people asking questions in response to a logline because they don't understand it -- you want them asking questions because they're interested to know more.

I was thinking of how Don Juan describes an agreed reality that didn't used to exist when wizards had their day but looking at it now it just doesn't make any sense. Maybe I could say 'stop him from becoming a powerful evil wizard' or 'super being' or something like that.

Hello, "destroying reality" I can't understand what it means, I should put something more concrete.