The guest right is an ancient and sacred tradition, that goes back thousands of years in Europe to the arrival of the Empire of Numeron, and its one of their most lasting cultural imprints on Europe. The Guests Right is practised in every single European kingdom of note, and it is this universal fact that makes it work, as if some places didn't then they could violate it without having any problems. The Guest Right has become very popular for being a way for two rivals to be able to talk without seeming weak, and for a weakened lord to be able to surrender without fear of death or violence against them.
The guest right is a sacred law of hospitality brought to the continent of Europe by the ancient Numenorians. When a guest, be he common born or noble, eats the food and drinks the drink off a host's table beneath the host's roof, the guest right is invoked. Bread and salt are the traditional provisions.
When invoked, neither the guest can harm his host nor the host harm his guest for the length of the guest's stay. For either to do so would be to break a sacred covenant that is believed to invoke the wrath of the Gods both old and new. Both the teachings of the old gods, Chrisitianity, Dragonoph and the Faith of Sigmar hold to this. Even robber lords and wreckers are bound by the ancient laws of hospitality.
The Guests Right has been broken many times by small lords, and by cravens, but in terms of large scale breaks in the Guests Right there have been only a few in the history of Europe as all the religions of Europe have it built within themselves, and thus to break it would be to incite the wrath of the gods.
The Bloody Supper was an event that was caused following the coruption of John Lovie by Sauron, and would lead to the deaths of dozens of noteable lords as well as the King of Lucerne, and his dragon. The Bloody Supper would lead to a massive civil war in Lucerne, and the destruction of the relationship between the Dragons of Lucerne as well as thousands of deaths.