Complex sentence: Jeanne, enraged by her husband's execution, swore retribution against the French King and Charles de Blois, she considered this a cowardly murder and swore she would avenge her husband.
Split into simpler sentences: Jeanne, enraged by her husband's execution, swore retribution against the French King and Charles de Blois. Jeanne was furious at what she considered a cowardly murder and swore she would avenge her husband.