"The unholy offspring of lightning and death itself"

(sort of a part 2 to my previous post about the healing thing)

In the first movie, Night Furies are described as being "the unholy offspring of lightning and death itself", but where does the lightning come from?? If it weren't for that quote, I doubt that Toothless would be associated with lightning at all.

If the guy who wrote the original book of dragons (I can't remember his name) witnessed the lightning ability we see in THW, surely it would be noted down in the book, hence the lightning reference, but it's not even mentioned until the third movie. The closest thing we get to Toothless having anything to do with lightning (before thw) is when he's in the clouds fighting the Red Death and his plasma blasts looks like lightning from below. Though I don't think that's the reason for it as we mainly see toothless swoop down from the sky to shoot and then sort of fly over the target, which all happens below the clouds.

Anyways, I'm hoping for some other thoughts on this :) (Sorry if I've got anything incorrect)