GURPS: Casus Belli
Rome is falling. Barbarians, once cowed by the might of the legions, now press in from all sides. The great city itself has been sacked, not once but twice. Territories once ruled by an iron fist have been abandoned or broken free.
In Prydain, nearly fifty years has elapsed since the legions withdrew. Territorial rivalries, once held in check by fear of Rome, have flared up into open war. Invaders – the Saecsen from Europe, the Scoti from the west, the Picts from the north – raid indiscriminately, killing and raping and plundering with no fear of retaliation. And the High King of Prydain, Wortigernos ap Guortheneu, is dead, leaving a dangerous vacuum of power waiting to be seized by anyone strong enough to hold it.
The year is now 463 Anno Domini.
And the land cries out for a king.
Casus Belli is a (relatively) low-fantasy campaign set in 5th Century Prydain using GURPS: Camelot and GURPS: Celtic Myth. GURPS: Imperial Rome, while not quite as extensively used, is also frequently referred to. Utilization of the Kingdom Building rules by Paizo for Pathfinder has also been implemented so the Warband may rule over their lands.