Thor, perhaps - it looks like it's just lightning filmed with a camera that doesn't capture the color very well, and then slowed down.
Discussion
The cloud sinking down with the bolt is a bit weird though.
I think I figured it out. The air is probably supersaturated with moisture, just waiting for a reason to condense out. The sonic shockwave from the lightning tips it over that limit, forming more cloud - it's not the cloud sinking down, it's a new part of the cloud.
I think you’re right.