That theory actually makes sense.
The moment you kill someone you can use that crush your soul in half and create a real shadow clone called Horcrux.
He used that to seal the kyuubi since it has a soul.
The drawback is that you turn into a grayish snakeman and that your name cannot be named anymore.....thus becoming Al.
It's a forbidden scroll cause it contains forbidden jutsu. Isn't that enough?
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"