Complex sentence: The unity of Kievan Rus'gradually declined, and by 1136 Kievan Rus' had shattered into a number of smaller states, the southern of which contested control of Kiev.
Split into simpler sentences: The unity of Kievan Rus'gradually declined, and was all but gone by 1136. After that period Kievan Rus' shattered into a number of smaller states, the southern of which contested for the throne of Kiev.