Sister blade to Widow's Wail, Oathkeeper is a Valyrian steel sword forged from Ice, the traditional weapon of House Stark. Initially, Tywin Lannister bestows the sword to his son Jaime to establish a new ancestral sword to House Lannister. The blade is notable for its hilt fashioned after a golden lion with ruby eyes.
Jaime reconsiders the gift in the wake of his son's (King Joffrey) death and presents the sword to Brienne of Tarth for the task of rescuing a missing Sansa Stark, whom Joffrey once terrorized.
In the seventh season, Sansa sends Brienne and Podrick to King's Landing as her proxies during the armistice meeting in the Dragonpit with Queen Cersei Lannister. Oathkeeper remains by Brienne's side during the meeting, but she hides the weapon from the eyes of the Queen who still has no idea Brienne has possession of the valuable Lannister heirloom.