I think I've seen that video in a nature documentary a couple of years ago. It was a documentary about the great extinctions that have happened during the history of Earth, and how nearly all life has been wiped out already five times, if I remember correctly.
Anyway, the video is a rendering of what would happen if a 500km asteroid would hit earth in the middle of the Pacific Ocean. Pretty much total devastation is what would happen, and the scale of the devastation is such that it is hard to comprehend. For example, you know what a tsunami is, right? Well, the impact would cause a tsunami, but not of water, of the earth shell. In the beginning of the video you see the japanese islands wiped out by the earth shell tsunami. They simply wouldn't exist after it.
Another thing is that the kinetic energies of the impact are so vast that a blast wave of boiled rock would traverse around the world, and it would not only incinerate everything it meets, but all surface water on earth would boil and disappear. All. Including all oceans. There would not be a drop of water on the surface of earth.
Impressively though, the scientist theorize that some bacteria could survive the impact and the time it takes for earth to cool down, but those are bacteria that live kilometers underground. So, I guess if an asteroid impact like that were to happen, humanity might be able to survive in some really sturdy, self contained underground dwellings, though what would be the point of that I can't say, since there wouldn't be any other life left on the planet save for the aforementioned bacteria... but it might make a cool setting for a game ;)