Arthur_I,_Duke_of_Brittany

Arthur I (Breton: Arzhur 1añ; French: Arthur 1er de Bretagne) (29 March 1187 – presumably 1203) was 4th Earl of Richmond and Duke of Brittany between 1196 and 1203. He was the posthumous son of Geoffrey II, Duke of Brittany, and Constance, Duchess of Brittany. His father, Geoffrey, was the son of Henry II, King of England.
In 1190 Arthur was designated heir to the throne of England and its French territory by his uncle, Richard I, the intent being that Arthur would succeed Richard in preference to Richard's younger brother John. Nothing is recorded of Arthur after his incarceration in Rouen Castle in 1203, and while his precise fate is unknown, it is generally believed he was killed by John.