Just watching this completely makes me think of D&D. Maybe it's just me...
I think that what actually happened is that from very early on a lot of various rock and metal bands actually had rpg players and often entire parties in them or friends of them which resulted in both parodies and serious pieces relating to rpgs from the very beginning of that lyrical trend. There was also a very strong book, comic, and movie fantacy presence. Conan for instance. All this stuff created a feed back loop and classic rock and metal was involved very early.Ok, I've heard of 'Spinal Tap'. I had a friend who told me about this really funny parody.
But it seems to have inspired a genre or something?