If you're ever, for some reason, trapped on a desert island populated by natives who have had no contact with the outside world and you need to convince them that you have the power to open the gates of hell, we've got a tip for you.
Known as the Pharaoh's Serpent, this nifty little trick uses mercury thiocyanate that, when ignited, produces a growing column of ash that looks like a tentacled creature from an H.P. Lovecraft novel. The fireworks demonstration is actually decades old, and apparently quite dangerous due to the toxic nature of the substance, but if you've never seen it before, it looks like a horrible vision from a nightmare that cannot be unseen. You can learn more about the science of how the reaction works here.
After you've made sure to turn all your lights on and cut off any potentially disturbing background music (dubstep, creepy movie soundtracks, etc.) in your home, you can take a look at a the Pharaoh's Serpent in action in the video below.