The historical document signed in 1215 is the Magna Carta, also known as the Great Charter. This significant charter was agreed upon by King John of England and a group of rebel barons, aiming to limit the power of the monarchy and establish certain legal rights for subjects. The Magna Carta is regarded as a foundational document in the development of constitutional law and the principle of due process. It emphasized that even the king must adhere to the law, laying the groundwork for modern democracy and influencing legal systems worldwide.