Complex sentence: The result of this experimentation 20 years ago calls a demon to Sunnydale that possesses Jenny Calendar by "Eyghon", which chastens their relationship; at the end of the episode, she tells Giles she needs some time away from him.
Split into simpler sentences: The result of this experimentation 20 years ago calls a demon to Sunnydale that possess Jenny Calendar. Her possession by "Eyghon" chastens their relationship; at the end of the episode, she tells Giles she needs some time away from him.