Arthur of Brittany
Arthur of Brittany (1187-1202) was the posthumous son of Geoffrey of Brittany, the fourth son of King Henry II of England. He was the grandson of Henry and under succession protocol of the time was the rightful successor to Richard I. (Exactly the same principle would be used when Richard II succeeded his grandfather Edward III in 1377.)
Richard’s younger brother, John, claimed the throne and this was supported by John's mother,Eleanor of Aquitaine (with the considerable forces of her dominions), the much respected William Marshal and many of the English barons. Arthur, through his mother (he was only 12) had an inevitable ally in King Philip II of France, who sent troops for the cause. The usual Angevan fracas ensued (the family had been at it on and off for over 25 years) with the usual French involvement.
Whilst the French and Arthur were besieging the 80 year old Eleanor in Mirebeau Castle, John’s forces captured Arthur in a raid on his camp. He was taken to one of John’s castles and never seen again.
Reference; The Plantagenet Encyclopedia