Complex sentence: Charles VI's friend and advisor, Olivier de Clisson, had recently been the target of an assassination attempt by agents of John V, Duke of Brittany; the would-be assassin, Pierre de Craon, had taken refuge in Brittany.
Split into simpler sentences: Charles VI's friend and advisor Olivier de Clisson had recently been the target of an assassination attempt by agents of John V, Duke of Brittany. The would-be assassin, Pierre de Craon, had taken refuge in Brittany.