Are you afraid of trolls? Vampires? Matrices?
For your Halloween pleasure, Matt Parker takes on all three fearsome concepts in the following 12 minute video:
If you're looking for more math to be scared by, since it was referenced in the video, here's Numberphile's five-and-a-half minute video on Vampire Numbers.
It's amazing how dated all the featured vampires other than Count von Count look in the videos....