I have a feeling this elder Yue is related to Ouyang, and is in charge of looking for new test subjects for him. As some people previously speculated, there must be some reason why the elves are having more and more negative feelings. Someone by the king's side could easily influence their thinking and maybe that, in turn, influences the tree of life and the feelings it allows the elves to experience. Basically ouyang seems to be trying to make a new devil world, then they can then help him open the seal? Since there is no dark matter, even if he is the devil king, since he was previously injured by Asura (aka Xiu), his strength should be quite low and he cannot upgrade without the dark matter. Just my guess anyway