Thunderous Ordeal

A dark thundercloud appears in a 20-foot high, 30-foot-radius cylinder centered on a point within range. The cloud never extends below 20 feet above the ground, but spreads around corners and is Heavily Obscured.
Any creature inside the cloud or vertically below is exposed to lightning from the cloud, unless the creature is immune to lightning damage or is separated from the cloud by a foot of non-conductive material (such as stone, wood or dirt).
Whenever an exposed creature performs an Action or Bonus Action, it must roll a d6; on a 1 or 2, it is hit by lightning and must succeed on a Constitution saving throw or be electrocuted. As soon as a creature has expended more than 5 feet of movement during its turn, it must likewise make a roll. At the end of the creature's turn, it must roll if it hasn't already this turn. Creatures resistant to lightning damage roll a d20 instead. Creatures with a weakness for lightning damage roll a d4.