Complex sentence: His death is attributed by sources used by John Tzetzes to another mortification of the same nature: in this case, the two soothsayers, jealous of each other's fame, came to a different trial of their skill in divination.
Split into simpler sentences: His death is attributed by some to another mortification of the same nature. The two soothsayers, jealous of each other's fame, came to a trial of their skill in divination.