Eustace the Monk was a younger son of a noble family in Boulogne. His moniker came from his youth in a Benedictine monastery. According to his legend, he served the count of Boulogne as a seneschal and bailiff but was accused of mishandling his stewardship. Eustace fled and was declared an outlaw. When the count confiscated his lands and fields, he burned two mills in retaliation. He became a mercenary pirate in the English Channel and the Straits of Dover.
Eustace the Monk sold the services of his fleet to the highest bidder. 1202 - 1212 he served King John of England in his war against Philip II of France, raided the Normandy coast and founded his bases in the Channel Islands, with John's acquiescence. When he raided English coastal villages too, John briefly outlawed him but then pardoned him because he needed Eustace's services.
Eustace switched sides in 1212 and raided Folkestone when English troops seized his Channel Island bases. When civil war broke out in England in 1215, he helped the rebel barons and the French invasion attempt.
In 1217 when Eustace the Monk and his fleet were transporting French troops to England, they met an English fleet. In the ensuring Battle of Sandwich, August 24, 1217, Eustace operated as Louis' admiral and wrought havoc among his former friends, the English, until the English blinded the French with powdered lime. English troops boarded his ships and defeated his men in melee. Eustace the Monk was beheaded on the spot.
A 13th century vernacular romance that relates his deeds, Li Romans de Witasse le Moine has recently been translated into modern English by Dr. Glyn Burgess. The contemporary historian Matthew Paris gives more detail of Eustace's career, in his Chronica Maiora (The Main Chronicle).
Eustace the Monk is not to be confused with Saint Eustace, the monk (died 625 CE), a favorite disciple and monk of Saint Columbanus, whom he succeeded as second abbot of Luxeuil in 611.