Complex sentence: After Francis's death Leo took a leading part in the opposition to Elias of Cortona: he it was who broke in pieces the marble box which Elias had set up for offertories for the completion of the basilica at Assisi.
Split into simpler sentences: After Francis's death Leo took a leading part in the opposition to Elias of Cortona. It was Leo who broke in pieces the marble box which Elias had set up for offertories for the completion of the basilica at Assisi.