Complex sentence: After leaving Yuan Shu, Lü Bu then headed to northern China to join Yuan Shu's relative Yuan Shao, and he assisted the latter in attacking Zhang Yan at Changshan (常山; around present-day Shijiazhuang, Hebei).
Split into simpler sentences: After leaving Yuan Shu, Lü Bu then headed to northern China to join Yuan Shu's relative, Yuan Shao. He assisted Yuan Shao in attacking Zhang Yan at Changshan (常山; around present-day Shijiazhuang, Hebei).