In September 1367 the pretender Enrique de Trastamara began his reconquest of the Castilian throne by crossing the Pyrenees, by way of the Val d'Aran, with a force of 400 lances. One of his principal followers was Bernard, called ‘the Bastard of Béarn’. Upon reaching Calahorra, where the small army was increased by such distinguished recruits as the Archbishop of Toledo and the Master of Alcantara, Bernard was knighted by Enrique's own hand. When the invasion, with the eventual help of the Breton captain Bertrand du Guesclin, had achieved its goal, and Enrique II, having killed his half-brother Pedro I ‘the Cruel’, once again wore the crown of Castile, the Bastard of Béarn was one of the first recipients of the royal largesse wryly known in Spanish annals as las mercedes enriqueñas. In July 1369 Bernard was created Viscount of Medinaceli, with considerable landed estates and privileges. Soon elevated to the rank of Count, he was given the hand of Isabel de la Cerda, a granddaughter of Alfonso X ‘el Sabio’ of Castile. This marriage of a Béarnais bastard with a lady of the royal blood of Castile was the origin of the still extant ducal house of Medinaceli.

Most modern authors – including Kervyn de Lettenhove, editor of Froissart's Chroniques – identify this Bernard de Béarn as a son of Gaston III Fébus; Pierre Tucoo-Chala, Fébus' most thorough and scholarly biographer, concurs. Russell inexplicably identifies Bernard with ‘Arnaud-Guillaume’ [sic] de Béarn, bastard brother’ of Gaston III.