He went on the First Crusade in 1096 with his brothers Godfrey of Bouillon (duke of Lower Lotharingia) and Baldwin of Boulogne. He soon returned to Europe to administer his domains. He married Mary of Scotland, daughter of King Malcolm III of Scotland, and Saint Margaret of Scotland. Eustace and Mary had one daughter, Matilda of Boulogne.
When his youngest brother king Baldwin I of Jerusalem died in 1118, the elderly Eustace was offered the throne. Eustace was at first uninterested, but was convinced to accept it; he travelled all the way to Apulia before learning that a distant relative, Baldwin of Bourcq, had been crowned in the meantime. Eustace returned to Boulogne and died about 1125.
On his death the county of Boulogne was inherited by his daughter, Matilda, and her husband Stephen de Blois, count of Mortain, afterwards king of England, and at the death of Matilda in 1152 it was inherited by their son, Eustace IV of Boulogne, later their second son William and ultimately by their daughter Marie of Boulogne, since both sons died without children (wikipedia.com)