Five rulers - the King of Ireland, the King of Denmark, the King of the Valley, the King of Sorelois, and the King of the Distant Isles - who waged war against Arthur after the establishment of the Round Table.
And as King Arthur rode to Camelot, and held there a great feast with mirth and joy, so soon after he returned unto Cardoile, and there came unto Arthur new tidings that the king of Denmark, and the king of Ireland that was his brother, and the king of the Vale, and the king of Soleise, and the king of the Isle of Longtains, all these five kings with a great host were entered into the land of King Arthur, and burnt and slew clean afore them, both cities and castles, that is was pity to hear.
Alas, said Arthur, yet had I never rest one month since I was crowned king of this land. Now skall I never rest till I meet with those kings in a fair field, that I make mine avow; for my true liege people shall not be destroyed in my default, go with me who will, and abide who that will.
Guenevere - The Invading Kings | The Legend of King Arthur