Bermudo III of Leon

Bermudo III ascended to the throne as a minor after the death of his father Alfonso V at Viseu. His minority was marked by regency and internal strife, as nobles vied for power. This period of weak central authority allowed the County of Castile to assert its independence.

Bermudo III lost control of the County of Castile when his sister Sancha married Ferdinand I, who then claimed the county as her dowry. This transfer of power strengthened Castile and weakened Leon, setting the stage for the eventual Castilian dominance over Leon.

Bermudo III was killed in the Battle of Tamaron while fighting against his brother-in-law, Ferdinand I of Castile. The battle was a decisive Castilian victory, and Bermudo's death ended the Perez dynasty of Leon. Ferdinand I then claimed the Leonese throne, uniting Leon and Castile.