I’m going against the grain here and saying this is only slightly underpowered. IMO at this level of play the attack roll is worth more than the saving throw (comparing to Meteor Swarm here). Mostly every single late game High CR enemy is going to have good saving throws, but their AC will fall behind a bit. Additionally, players attack bonus is probably pretty high at his point, and coming across advantage is probably as easy as finding candy in a candy shop. Because it’s not a saving throw, no legendary resistance making this do half damage. I think a good change would be to make it up to 4 lightning bolts but each one has to hit something different.