Complex sentence: Fearing for the safety of the boy, Helias took him to the court of Louis VI of France, who joined the discontented Norman barons and other enemies of Henry I in recognizing William as the rightful claimant to the duchy.
Split into simpler sentences: Fearing for the safety of the boy, Helias carried him to the court of Louis VI of France. That sovereign joined with the discontented Norman barons and others of Henry's enemies in recognizing William as the rightful claimant to the duchy.