Do those lightning things really turn grand soul gems to black ones? And do they really only work once? If so then what the heck is the point of them? Also, do monsters in the soul cairn respawn? Since they do drop a large amount of soul gems lol.

the creatures do respawn
and yes those rods turn them into black soul gems
not sure if they reset tho

Awesome. Thanks!

The lightening rods do not reset, you only get one go. But you can still come back with a lot of soul gems on the second trip, or even a ton of enchanted armour and weapons. All the enchanted weapons hidden around the place will respawn, and the soul husks all respawn if you want to use them to trade for stuff.

You don't even really need grand soul gems for the lightning rods. It took Greater soul gems out of my inventory and turned them into Black Soul gems. As Aussie already mentioned, unfortunately they do not reset. However, I did pick up a few Black soul gems off some skeleton mobs, during my 2nd trip.