In September 1101, his entire army was destroyed by the Seljuk Turks led by Kilij Arslan I at Heraclea; William himself barely escaped, and, according to Orderic Vitalis, he reached Antioch with only six surviving companions.
Pada bulan September 1101, seluruh pasukannya dihancurkan oleh orang-orang Turki Seljuk yang dipimpin oleh Kilij Arslan I di Heraklea; Guillém sendiri nyaris lolos, dan, menurut Orderic Vitalis, ia mencapai Antiokhia hanya dengan enam sahabat yang masih hidup.