House of Trastámara
In 1388, as part of the Treaty of Bayonne, he married Catherine of Lancaster in Palencia Cathedral. She was the daughter of John of Gaunt, the Duke of Lancaster, and Constance of Castile, a daughter of Peter the Cruel. This solved the dynastic conflict that had raged since the death of Peter the Cruel, secured the House of Trastámara, and established peace between England and Castile.