Do you make anything of the Valyrian prophecy (from WoIAF) that Lannister gold would bring about the Doom? I mean, the Lannisters bought their sword right before the Doom, and “Brightroar” is about as succinct a description of the Doom as one can get.
“Right before the Doom” is a highly suggestive statement. Brightroar was bought a century before the Doom. As were who knows how many other Valyrian swords (of the 227 currently existing in Westeros). As TWOIAF says:
Perhaps in preparation for their crossing of the narrow sea, the Valyrians also established their westernmost outpost on the isle that would come to be known as Dragonstone some two hundred years before the Doom. No king opposed them—and though the local lords of the narrow sea made some effort to resist it, the strength of Valyria was too great. With their arcane arts, the Valyrians raised the citadel at Dragonstone. Two centuries passed—centuries in which the coveted Valyrian steel began to trickle into the Seven Kingdoms more swiftly than before—though not swiftly enough for all the lords and kings who desired it.
–The World of Ice and Fire
And note that Ice was also bought around a century before the Doom of Valyria:
Catelyn had no love for swords, but she could not deny that Ice had its own beauty. It had been forged in Valyria, before the Doom had come to the old Freehold, when the ironsmiths had worked their metal with spells as well as hammers. Four hundred years old it was, and as sharp as the day it was forged. The name it bore was older still, a legacy from the age of heroes, when the Starks were Kings in the North. –AGOT, Catelyn I
As for Brightroar’s name: Nightfall, Lamentation, and Red Rain are also sword names that are pretty good descriptions of the Doom. Heck, even Blackfyre sounds a bit like a volcano erupting ash and flame, doesn’t it. Threats of war and death, fire and blood; most of these Valyrian steel sword names want to tell the same kind of story.
Also, you’ve got the wording of the prophecy wrong, sorry.
The wealth of the westerlands was matched, in ancient times, with the hunger of the Freehold of Valyria for precious metals, yet there seems no evidence that the dragonlords ever made contact with the lords of the Rock, Casterly or Lannister. Septon Barth speculated on the matter, referring to a Valyrian text that has since been lost, suggesting that the Freehold’s sorcerers foretold that the gold of Casterly Rock would destroy them. Archmaester Perestan has put forward a different, more plausible speculation, suggesting that the Valyrians had in ancient days reached as far as Oldtown but suffered some great reverse or tragedy there that caused them to shun all of Westeros thereafter.
–The World of Ice and Fire
Now, the fact that Septon Barth is discussing the subject is a general clue that he was right and we should ignore whatever the skeptic Yandel says. But note that the prophecy does not say “Lannister gold will bring about the Doom of Valyria”, it says “the gold of Casterly Rock would destroy them” – “them” meaning Valyrians.
So it might be that the gold from the purchase of Brightroar – or any other Lannister gold from any other trade – made its way into the hands of those who paid the Faceless Men to assassinate the sorcerers who controlled the Fourteen Flames, and so brought about the Doom. Or it might be that this prophecy refers to Tywin’s part in Robert’s Rebellion, and especially Jaime’s assassination of Aerys Targaryen (remember that Jaime was wearing his Lannister gold armor at the time). Or it might be that the prophecy refers to something that hasn’t even happened yet – and do recall that Tyrion has promised over 150,000 golden dragons of Casterly Rock to the Second Sons for allowing him to join them.