What would you do if you were walking down the street and saw this happen?
Well quite.
What if you were sitting on a bench, and the guy next to you claimed he couldn't see the figure standing over you?
And then this happened?
Maybe you'd adopt a sort-of-punch stance, like this?
And at night the overall effect is so scary it makes people trip over benches.
Ah, the most deadly of all Harry Potter curses.
Watch the full video here:
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Point of order: It's quite possible the people who made this video and titled it a "Dementor" prank have never seen a Harry Potter film before.