As he was growing up, Deun Yu probably expected that someday, as the nephew of reigning Dali King Deun Jing Ming and the only son of Prince Deun Jing Tsun, he would ascend to the Dali throne himself (which indeed happened). Since Deun Yu's real father was Deun Yin Hing, not Deun Jing Tsun, however, wouldn't this have complicated things?
Originally, Deun Yin Hing was the legitimate heir to the Dali throne. Due to civil strife in Dali and DYH's subsequent activities as leader of the Four Great Evils, I'm pretty sure that Deun Yin Hing had been discredited as a potential Dali monarch. If so, did Deun Yu really have a legitimate claim to the Dali throne? What if it got out that his real father was not Deun Jing Tsun, but Deun Yin Hing? Many supporters of Deun Jing Ming and Deun Jing Tsun who opposed Deun Yin Hing might find it difficult to accept Deun Yu as ruler after that. Moreover, Deun Yu was born out of wedlock: his mother was not married to Deun Yin Hing when Deun Yu was conceived. Somehow, however, none of this ever became an issue in DEMIGODS & SEMIDEVILS (possibly because there was a cover-up?).