His Proposal

With just that question alone, Rui's sharp mind was already able to surmise the strategy that she had in mind. 

Light bending was the most important ability and measure of blood purity with the Sun God Solaris. It was so important that it decided who the ruler was going to be, and it served as the scale by which sociocultural and even Martial hierarchy was decided.

Although the combat power of a Martial Artist was not solely decided by light bending, because the culture was so engrossed in light and light bending, almost all their Martial Artists integrated light into their Martial Art. Thus, the strongest Martial Artist was also regarded as the strongest light bender and thus queen.