William of Saint Omer was a French knight, descended from a Fauquembergues family who were castellans of the eponymous castle of Saint-Omer.
[1] His father, Nicholas I of Saint Omer, received lands in Boeotia in the aftermath of the Fourth Crusade.
[4] William was the younger son of the couple, he had an elder brother Bela of Saint Omer.
He participated in the Battle of Mohi, following this he belonged to the companion of Béla IV who had fled to Dalmatia, escaping before the Mongols.
According to his epitaph at the Cathedral of Saint Domnius, William died in April 1242 in Trogir, where the royal family resided until the Mongol occupation.