Chilling. Horribly believable, given the basic background that's all I know of World War Z. The style certainly fits the crossover, but you're left there wishing that things will work out for those left behind.

The only trouble is I can't quite see Buffy leaving all her fellow surviving Slayers (including Faith) behind that that. Certainly not willingly. Maybe it was something somebody did to her, rather than her walking away. Sort of thing I can see Dawn doing...

And no mention of any of the witches connected to the Council/Slayers? Willow? She would have been rather a large gun for them.

Still, loved it. Kept me anxiously reading once I got into it - and like I said, I don't know the original. Excellent work.