Complex sentence: In 1345 tragedy struck, Bianca's younger brother John died, this left only one son and heir to Savoy, Bianca's older brother Amadeus VI, Count of Savoy.
Split into simpler sentences: In 1345 tragedy struck when Bianca's younger brother John died. This left only one son and heir to Savoy, Bianca's older brother Amadeus VI, Count of Savoy.