[2][3] On the death of Peter of Castile in 1369, Ferdinand, as great-grandson of Sancho IV by his grandmother Beatrice, laid claim to the vacant Castilian throne.

The kings of Aragon and Navarre, and later John of Gaunt, Duke of Lancaster, who had married Peter of Castile's eldest daughter, Constance, also claimed the throne.

However, the outward concord was soon disturbed by intrigues with John of Gaunt, Duke of Lancaster, brother of Edward the Black Prince, who entered into a secret treaty with Ferdinand for the expulsion of Henry from his throne.

In 1383, the Treaty of Salvaterra stipulated that Beatrice, Ferdinand's daughter and heiress, would marry King John I of Castile,[7][8] and thus secure the ultimate union of the two crowns.

[4] Ferdinand left no male heir when he died, probably from poisoning, at Lisbon on 22 October 1383,[9] and the direct Burgundian line, which had been in possession of the throne since the days of Count Henry (about 1112), became extinct.