I saw it a few betas back, as well. Nasty event, even killing off a group of level 9 champions. Went back to my last saved game to get around it. The surprising thing to me was that as long as I tried this out, none of the AI players were destroyed--because the spiders did destroy two of my cities.
The kind of event that's largely unsuccessful, I think, because it simply causes players to reload to avoid it.