Duke Arthur I of Brittany was born in 1187 and became the Duke of Brittany in 1196. He was the son of Geoffrey Plantagenet, the Duke of Brittany, and Constance of Brittany. Arthur was a key figure during the late 12th century, known for his claim to the English throne through his mother, Constance, who was a granddaughter of Henry II of England. Arthur's life was marked by conflict, particularly with his uncle, King John of England. In 1203, Arthur was captured and imprisoned by John, leading to his mysterious disappearance in 1204. His fate remains uncertain, but he is remembered as a symbol of Breton resistance against English rule.