I'm pretty sure at some point in the past I read a Conan story (could be from Howard or a later author) which features a scene something like the following: Conan leads an army against an opposing force. Before the battle is joined, an enemy wizard-type drives across the field of battle in a chariot, spilling what seems to be a chemical trail in a line from the back of it. Subsequently, this line springs into explosive fire as cavalry attempt to cross it.
What story is that (and chapter)?