Its not even a kilometer in size. It'll fuck up wherever it hits something fierce. Like, up to two three hundred miles would just be blasted away or severely damaged. Depending on composition, lots of variables but this is worst case. Maybe we will drop a degree or two, a year without summer. With current manufactured food shortages it would be rough for quite a few people. We would survive it tho. Maybe a billion die, two if we are lucky. But it's hardly a world ender.
The one that hit 65mya would be very difficult to survive unless you have a serious bunker, and decades of food or a way to grow food without any sunlight for a long time, then deal with world wide crippling freezing for a century or two, then ya might make it.
The one the size of Armageddon the movie. No hope, nothing alive today will survive. The entire surface of the planet would be turned to a molten hell scape. Not even the hardiest of waterbears would make it.
Maybe tho. If some of the ejecta has microbes on it, and they stay in orbit long enough for the earth to cool, and rain back down...maybe. Maybe something of what we know today would survive and be reborn hundreds of millions of years from now.