King John I of Castile was a monarch who ruled from 1379 to 1390. He was the son of Henry II of Castile and Leonor of Aragon. His reign was marked by political instability and conflicts with the nobility, which often challenged his authority. John I is also known for his marriage to Beatrice of Portugal, which strengthened ties between Castile and Portugal. Despite his efforts to consolidate power, his reign faced difficulties, including economic issues and military challenges, ultimately leading to a legacy of a weakened monarchy.