The king known for signing the Magna Carta in 1215 was King John of England. This landmark document was created in response to the king's heavy taxation and arbitrary rule, stemming discontent from nobles and clergy. The Magna Carta established the principle that everyone, including the king, was subject to the law, laying the foundation for the development of constitutional governance and the protection of individual rights. It is considered a seminal document in the history of democracy and legal rights.