Brightroar came into the possession of the Kings of the Rock from House Lannister in the century before the Doom of Valyria, and it is said that the weight of gold they paid for it would have been enough to raise an army.
Lancel IV Lannister is said to have beheaded the ironborn king Harrald Halfdrowned with Brightroar, and he wielded his Valyrian sword against Ser Wilbert Osgrey. This seems to be another sword with the same name, since Lancel IV Lannister's contemporary, King Gyles III Gardener lived at least a thousand years before the Conquest.
Brightroar was lost little more than a century later, when Tommen II Lannister carried it with him when he sailed with his great fleet to ruined Valyria, with the intention of plundering the wealth and sorcery he was sure still remained. The fleet never returned, nor Tommen, nor Brightroar.