Complex sentence: He was given a long bow to carve a notch into whenever he performed a good deed and when attaining 100 notches would be judged truly worthy of having taken the mantle of the Black Marvel.
Split into simpler sentences: He was given a long bow into which he carved a notch whenever he performed a good deed. When he had attained 100 notches, would be judged worthy of having taken the mantle of the Black Marvel.