So first, if watching reruns of Aliens does not quench your desire to see horrifying creatures emerge from the abdomen of another, take a look at this.
There's really not a lot we can say for sure about this video. Somebody killed a praying mantis, maybe because he didn't like the way they treat their mates. And a some sort of parasite three times the length of the mantis decided it was time to skedaddle.
We're not taxonomists here, and there is no peer reviewed study to rely on, but commentators on Youtube (not normally our preferred source of reliable info) think it is a horsehair worm or Nematomorpha. If so, and it seems to be right, what you are about to read is arguably more horrific than the video itself. Nematomorph hairworms don't just colonize the belly of their hosts, they take over their brains as well. The hairworm makes grasshoppers jump into water – fatal for the grasshopper but a great move for the parasite, as they need water to reproduce. Such worms can grow to two meters long, although 50cm-1meter is more common. The adults live freely in water, but the larvae are parasitic on various insects and crustaceans. They are known to tie themselves in knots, leading to the nickname Gordian worms.