Years before the story takes place the Phoenix left behind some of its soul to pass down its legacy. This remnant soul was eventually used by the Divine Phoenix Sect who eventually took over the Grand Asura Kingdom renaming it the Divine Phoenix Empire.
Because of the corruption the Phoenix Soul was exposed to it was able to disobey the will of the Phoenix which should have been impossible.
After it became corrupt it could sense that another Residual Phoenix Soul existed in close proximity to it so in order to be the only one on the continent it went and tried to kill the other Residual Soul, but failed. During their fight the other Residual Phoenix Souls World Ode of t he Phoenix was destroyed but the other Soul also broke apart the Phoenix Gods own World Ode of the Phoenix leaving it with only the first four stages. 
Sometime later it became lucid and regretted what it did to the other Phoenix Soul and before its death it left its entire being for Feng Xue'er.