Complex sentence: The emperor had killed the court mystic preceding Luan Da, Shaoweng (who had studied with the same teacher as Luan Da), by poisoning; purportedly by horse liver, which was thought to be poisonous at the time.
Split into simpler sentences: The emperor had killed Shaoweng, the court mystic who preceded Luan Da and who had studied with the same teacher as Luan Da. Shaoweng's death was purportedly by horse liver, which was thought to be poisonous at the time.