My various webcrawls took me to a potentially scary event that may or may not occur on April 13, 2036, though the odds are only 1 in 6250 that this will occur. As is usual, this number may change between now and then. I certainly hope it changes for the better. This eventuality is named Apophis, and should it impact, it will do so with a force of 880 megatons, or roughly 65,000 Hiroshimas. This asteroid is roughly 300 metres wide and has a mass of approximately 46,000,000,000 kg. Compare this to the Tunguska event, which was a meteorite that detonated with the force of 15 megatons, wiping out an area roughly the size of Rhode Island, though thankfully, with minimal loss of human life.
After doing a touch of research, I find the naming of this object oddly fitting. Apophis, alternately known as Apep, Apepi, or or Aapep, is the Egyptian deification of darkness and chaos, the opponent of Ma'at, and was represented by a snake. His identity was later completely subsumed by Set in a bit of nationalistic furor after the Middle Kingdom, so if you're more familiar with Set, basically, duplicate it.