Most legends about her in Chinese mythology involve some variation of the following elements: Ho Yi the Archer; an emperor, either benevolent or malevolent; an elixir of life; and the moon.
Chang E flew to the moon with the elixir of life, chased by her famous archer husband Ho Yi.
Many years after she was already the moon goddess, Cheng E looked down upon Earth and saw that a terribly cruel and tyrannical emperor sat on the throne. To help the people, she allowed herself to be reborn into the mortal world. The other members of her mortal family were either killed or enslaved by the emperor, but Chang E managed to escape to the countryside.
Meanwhile, the emperor was aging and obsessed with discovering the elixir of life. He had people all over the land brought to him and demanded of them how to find the elixir of life; nobody knew, of course, but the emperor would not accept ignorance for an answer and executed all those who could not answer.
In the countryside, Chang E met the goddess of compassion Guan Yin, who proceeded to give Chang E a small elixir. Chang E brought the elixir to the emperor. The suspicious emperor worried that it was poison and demanded that Chang E taste the elixir first. She did, showing no ill effects, so then the emperor took the elixir and promptly died. Then, Chang E also left the mortal world; the effects of the elixir had only been delayed for her. However, instead of dying, she ascended to the moon to retake her place as a goddess.
See also: Lunar deity, Mid-Autumn FestivalCheng E and Ho Yi the Archer
Cheng E and the Cruel Emperor