Are there any ancient battles people can recall that involve lots of fog or mist obscuring the battlefield? I'm thinking of designing a sort of hidden movement module for some systems that can allow units to sort of blunder into each other, with rolls for each section of the battlefield to clear each turns
Barnet (though that's medieval, I guess...).
Lake Trasimene (if it wasn't invented as an excuse). Deviation probably not an issue though, as the Romans seem to be following the lake.
Fog in the opening stages of the Crimeisos, though all it did was mask the Carthaginian approach march.
Maclodio 1427.