Complex sentence: The rites were based on a seasonal death-rebirth theme (common among agricultural cults) and spirit possession; the Osirian Mysteries paralleled the Dionysian, according to contemporary Greek and Egyptian observers.
Split into simpler sentences: The rites were based on a seasonal death-rebirth theme and on spirit possession. The Osirian Mysteries, for example, closely paralleled the Dionysian, according to contemporary Greek and Egyptian observers.