It was believed that the world always existed and was never changing, for nothing could ever exist without Law and Order. Angels of the World Above served the incredible power that is the Operator and watched over mankind, inhabitants of the World Below, with ever vigilant eyes. None would have suspected that it be one among them who would betray the very foundation of the known universe.
Ledsabbath, an angel of the highest rank, found the never changing notes of the divine choir boring and the mortal crowds' uniformity pathetic; he wanted something that was never conceived before, something new.
He knew that the lesser minds would be far easier to tempt, so he stole the divine spark, planted it into a musical instrument of his own design, and traversed the Void between the World Above and the World Below on the Black Zeppelin, a legendary vehicle.
Upon arriving, Ledsabbath sounded the instrument, now known as the Ultima Guitar, and lo: the chaotic powers let loose, like a virus, infecting and freeing the minds of mortals as they heard the pure awesomeness that is metal music. Mankind has broken its shackles of ignorance and chose to carve its own future.
The Operator and His angels tried to save their lost children, but their attempts so far have been fruitless. For his grave sin, Ledsabbath was cast into a pocket deimension and chained to a rock on an impossibly high mountain top, where a thirteen-strong angelic choir shall sing the painfully monotonous melodies only to him - forever.
Since that, the World Below has never been and shall never be the same again. Metal music has torn the very fabric of reality, springing forth mystic creatures and horrible abominations into existence. It is this new world in which you start your adventure.
I imagine this setting would perfectly work with the standard gameplay assumptions of D&D. I have already started working on how magic guitars function (more or less replacing magic swords, I guess), and figured this setting would require some unique dragons as well (possible a simple dragon generating procedure).