New clerical monster spell (possibly also a player spell) that creates a region of black storm clouds between the caster and target and moves across the map towards the target. The clouds block vision, can strike anything in or near them with lightning (though not intelligently), wet items on the floor beneath them and in the inventories of monsters beneath them, hit things with gusts of wind if those are implemented, and can spawn temporary lightning monsters that vanish when the cloud does.

Another variant of this idea is as a static cloud that can be targeted like a stinking cloud, probably as a level 7 spell (depending on how bad the lightning is).