Ugh, it was a kind of poison made from the resentful dead, infused with some of what made people puppets back in the war... the kind we ran into was powdered, we inhaled it in the fog.

( None of that sounds good, but he has no great way to explain when it's kind of a nebulous thing none of them studied because it wasn't supposed to still exist. (Xue Yang, as it turned out, had it in spades. )