Brightroar was the ancestral Valyrian-steel sword of House Lannister, borne by the Kings of the Rock and, after the Conquest, by the Lords of Casterly Rock. It came into Lannister hands during the Century of Blood that followed the Doom of Valyria, bought at a cost the family has never disclosed.
The blade was lost about a century before Aegon's Conquest, when King Tommen II Lannister sailed for the smoking ruin of Valyria with Brightroar at his side, hoping to wrest fresh treasures and Valyrian steel from the Freehold's grave. Neither the king, his fleet, nor his sword were ever seen again. The Lannisters have sought to buy back a Valyrian blade ever since, though no price has yet sufficed.
Status: lost since the voyage of Tommen II to Valyria.