The Kingdom of Brigantys, was a nation that existed on the Valerian Isles at least 50 years before the events of Tactics Ogre: Let Us Cling Together. The kingdom was based around Brigantys Castle, and ruled by King Rodrick Desmoria.
When Dorgalua's Great War swept across the isles, Brigantys was drawn into the fray after their neighboring ally, the Kingdom of Coritanae, was attacked by the Kingdom of Phidoch. Eventually, the state of the war was reduced to a long battle between two armies, with Rodrick on one side, and Dorgalua on the other. In his desperation, Rodrick used spells of the Apocrypha, causing widespread devastation to both armies, and even the landscape. This would inadvertently lead to Brigantys' defeat, as Rodrick's outraged people turned against him.
After the war, the Kingdom of Brigantys became part of the newly formed Kingdom of Valeria, under Dorgalua's reign.