Even though it looks like the devil is reaching through the ground from underneath the Earth to grab a victim, you can't look away. Even though it looks like some alien tentacle snake is burning itself and trying to attack you, you can't help but stare. It's the Pharaoh's Serpent (or Black Snake firework) and it's an old science experiment that's always so, so gross to look at. The disgusting, growing mutant arm coil thing is actually just Mercury thiocyanate (Hg(SCN)2) getting lit on fire.
It starts off as a white power but when heated up, "a rapid exothermic reaction is started which produces a large mass of coiling serpent-like solid." AKA that's why there's an ugly arse snake coming up from the ground.
I always liked lighting that Black Snake firework but this one is too real. You can learn more about the science here.