Oathkeeper is one of the two Valyrian-steel longswords reforged from Ice, the ancestral greatsword of House Stark, after Lord Tywin Lannister had the captured blade broken down at King's Landing. There was not steel enough in Ice to make three swords, so Tywin settled for two, each rippled in alternating bands of red and black where the old metal was folded and refolded under Tobho Mott's hammer.
Tywin meant the paler blade for his son Jaime, but Jaime, newly returned to King's Landing maimed and oath-bound, gave the sword instead to Brienne of Tarth. He charged her to find and protect Lord Eddard's daughters with it, and bade her name it; she called it Oathkeeper, for the vow it was meant to honour. Brienne has carried it through the riverlands ever since.
Status: extant, borne by Brienne of Tarth.