Brightroar

"A foolish King of the Rock took House Lannister's Brightroar to Valyria... neither man nor blade was seen again."

- Ser Jorah Mormont

Brightroar was the Valyrian steel blade of House Lannister. It was lost when Tommen II Lannister, King of the Rock, took it on his expedition to the ruins of Valyria and dissappeared.

The lack of a Valyrian steel sword for House Lannister drove Tywin Lannister to have Ice, the Valyrian steel greatsword of House Stark, reforged into two blades.

In the books
In the A Song of Ice and Fire novels, Brightroar was the ancestral Valyrian steel greatsword of House Lannister. House Lannister has been searching for a replacement ever since it was lost. Tywin Lannister tried numerous times to buy one from impoverished noble houses but was always rebuffed, lords preferring to offer their daughters in marriage rather than part from such valuable heirlooms. Meanwhile, Tywin's youngest brother, Gerion Lannister, went into an expedition to the Valyrian Peninsula aproximately in 291 AC, with the goal of finding Brightroar, and, like King Tommen II, never returned.