Brightroar
The blade the richest family in Westeros bought, and then lost so completely they have been shopping for a replacement ever since.
The blade itself
Brightroar was a longsword, and by the descriptions that survive it was a showy one: a lionÔÇÖs head worked into the pommel, gold on the fittings, the dark rippled Valyrian blade set off against as much Lannister ornament as could be attached without spoiling the balance. The steel itself would have been the usual smoke-grey shot with waves, catching the light in bands, taking an edge nobody ever had to put back. Everything else about it was the buyerÔÇÖs taste rather than the makerÔÇÖs.
Nobody alive has seen it. What is certain is what is certain of all such blades ÔÇö that it could not be reproduced, that no smith in Westeros could have mended it, and that the making of Valyrian steel died in the Doom along with the men who knew the spells. It follows that a family who lost one had lost it permanently, and that no amount of gold could commission another. That last point is the whole of BrightroarÔÇÖs significance, and House Lannister has never entirely got over it.
Bought, not won
The Lannisters did not take Brightroar off anybody and no ancestor wrestled it from a dragon. They bought it, during the century of chaos after the Doom, when the surviving Valyrian families in the free cities were selling everything that was not nailed down and a great many things that were. The price is recorded only as a comparison: it is said to have cost as much as raising and provisioning an army.
That is an unusually honest provenance for a Westerosi heirloom, and it is also very much in character. House LannisterÔÇÖs power has always run on gold first and swords second, and the acquisition of a Valyrian blade by simple purchase ÔÇö the only method available to a family with no ancient claim to one ÔÇö is the Rock in miniature. It also gave every rival house a joke to make at their expense, which may explain some of what came next.
Lost with the Valyria expedition
King Tommen the Second of the Rock took Brightroar east. He fitted out a great fleet and sailed for the Smoking Sea to plunder the ruins of Valyria itself ÔÇö a place that men had been sailing carefully around for a century, on the grounds that ships which go in do not come out. He was not deterred. Neither the king nor the fleet nor the sword was seen again.
The Lannisters have never satisfactorily explained the decision. The kindest reading is that a young king wanted a legend; the more likely one is that ValyriaÔÇÖs ruins were widely believed to be stacked with dragon gold and unclaimed steel, and that a family who bought their first Valyrian blade fancied collecting the rest for free. Whatever the reasoning, the result is that the wealthiest house in Westeros put its single irreplaceable possession on a boat and sailed it into the one part of the world nobody returns from.
The long shopping trip
What followed has a faint air of embarrassment about it. Successive Lannisters made discreet enquiries, and later much less discreet ones, about buying a Valyrian blade from any house that would part with one. None would. The great families with swords understood exactly what was being asked and enjoyed refusing it; a house that sells its Valyrian steel to Casterly Rock is a house announcing that it is finished. Tywin Lannister is said to have made offers to more than one impoverished lord and to have been turned down every time, which for a man of his temperament will not have improved matters.
There was also a second expedition, quieter than the first. Gerion Lannister ÔÇö TywinÔÇÖs youngest brother, the funny one, the one everybody liked ÔÇö took a ship east to look for the sword. He got as far as Volantis. Nothing certain has been heard of him since, which makes Brightroar the only object in this collection to have cost two members of the same family.
Understanding all that makes a certain later Lannister decision look less like spite and more like appetite. A house that has spent two centuries failing to buy Valyrian steel, presented with a rivalÔÇÖs ancestral greatsword and no reason to give it back, was never going to hang it on a wall.
What became of it ÔÇö if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.