Complex sentence: Under pressure of Charles Toka, the master of Kefalonia, the Despot demanded help from his brother John VIII. That help came in the form of reinforcements led by their brother Constantine, who became joint ruler of Morea with Theodore II in 1428.
Split into simpler sentences: Under pressure of Charles Toka, the master of Kefalonia, the Despot demanded help from John VIII. Bill for that help was brother Constantine, who became joint ruler of Morea with Theodore II in 1428.