Complex sentence: The result of this experimentation 20 years ago calls a demon named "Eyghon" to Sunnydale who possesses Jenny Calendar, chastening 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 named "Eyghon" to Sunnydale; the demon then possesses Jenny Calendar for a time, after which she chooses to halt the progress of their relationship. At the end of the episode, she tells Giles she needs some time away from him.