Bermudo III he was king of Asturias and León. He was born in 1009 he was the son of Alfonso V, whom he succeeded in 1027 . With Bermudo III, the male succession of the kings of Asturias and León was extinguished, and this kingdom joined Castile through the female line. In 1028, Bermudo III married Jimena Teresa (or Urraca), sister of the count of Castilla García Sánchez; he was going to marry Sancha (sister of Bermudo III); but, when he came to León to celebrate the wedding, he was assassinated by the sons of Count Vela from Alava on May 13, 1029 without the marriage taking place. This was revenge, because this count from Alava, Vela, had been dispossessed and expelled from Castile by Sancho Garcés (father of the murdered García Sánchez). After this murder, the county of Castile passed to the King of Navarre, Sancho Garcés III, by inheritance from his wife Elvira (older sister of the murdered García Sánchez).
Now, the kingdom of Sancho Garcés III bordered the kingdom of Bermudo III, and they soon came into conflict. Indeed, the city of Palencia had disappeared before Almanzor. Sancho Garcés III vowed to build a temple to Saint Antolín, where Palencia had been before, and rebuild this city. Bermudo III opposed this project, alleging that this territory corresponded to his kingdom; but the Navarrese assured that said territory belonged to Castile; so they fell out. Later, while Bermudo III was busy suppressing an uprising in Galicia, Sancho Garcés seized the territory between the Pisuerga and Cea rivers . Bermudo III came against the Navarrese; but, with the intervention of the Navarrese and Leonese bishops, they concluded peace, which, to be more firm, agreed to the marriage of Sancha (sister of Bermudo III, who had previously been engaged to the murdered García Sánchez, count of Castile) with Femando, son second of Sancho Garcés III of Navarre, who would cede, to his son Fernando, the county of Castile raised to a kingdom plus the territory between the Pisuerga and the Cea, which the Navarrese had appropriated. The marriage was celebrated in 1032. Thus the kingdom of Castile appeared, whose first kings were Ferdinand I and his wife Sancha. However, while Bermudo III was in Galicia repelling the Muslims, Sancho Garcés III again invaded the kingdom of León and seized Astorga; but, when the Navarrese died in 1035, Bermudo III recovered the territory that he had usurped from him, and also wanted to recover the lands between the Pisuerga and the Cea, which he had given to his sister Sancha when she married Fernando; reason why Bermudo III entered Castile with an army; but he was defeated and killed by Fernando with the help of his brother García Sánchez III (who had inherited the kingdom of Navarre in 1035) in the battle of the Tamarón valley , north of Palencia in June 1037. As Bermudo III had no offspring, his kingdom passed to his sister Sancha, who, being married to Fernando, king of the newly created kingdom of Castile, the incorporation of the kingdom took place from Asturias and León to the kingdom of Castile , leaving both kingdoms united from that moment under the sovereignty of Fernando I with the title of King of León.