Magna Carta
The Magna Carta (Latin: great charter) was the first document in British history to place legal limits on the powers of the King.
In 1215, the nobles of England, chafing under King John's excessive taxation and arbitrary justice, would only renew their oaths of fealty if John agreed to their demands. John was forced to agree, and all parties signed the Magna Carta in June 1215. It provided, among other things, that a free man could not be imprisoned or have his property seized without due process, and that he was entitled to a quick trial before his peers.
Though its importance has been exaggerated in the public mind, it is still an important document in the history of jurisprudence, and its signing, a pivotal moment in human history--no doubt why the Master sought to prevent its signing in The King's Demons.