The first monarch of the United Kingdom was King William I, also known as William the Conqueror. He ascended to the throne in 1066 after his crucial victory at the Battle of Hastings, which marked the Norman Conquest of England. William I established a new ruling dynasty, significantly altering the country's political, social, and cultural landscape. He implemented feudal systems, reorganized land ownership, and commissioned the Domesday Book, a pivotal record of landholdings. His reign set the foundation for the evolution of the English monarchy, influencing subsequent monarchs and the governance of England, which eventually contributed to the formation of the United Kingdom.