Complex sentence: Thus he decreed that Amin would rule the Caliphate unil his death (while Ma'Mun had his own virtually autonomous province in Eastern Iran) at which point Ma'Mun and his sons would continue to rule.
Split into simpler sentences: Thus he decreed that Al-Amin would rule the Caliphate unil his death. Al-Ma'Mun had his own virtually autonomous province in Eastern Iran which Al-Ma'Mun and his sons would continue to rule.