Oathkeeper is one of two Valyrian steel longswords made from Ice, the greatsword of House Stark. Its sister blade is Widow's Wail.

Tywin Lannister gave the sword to his son, Jaime, intending for him to be released from his vows as Lord Commander of the Kingsguard and rule over Casterly Rock in his stead. Although Jaime refuses, Tywin allowed him keep the sword anyway.

Following Joffrey's death, Jaime tasked Brienne of Tarth with finding Sansa Stark and getting her somewhere safe before his sister, Cersei located her. Cersei blamed Sansa for Joffrey’s death and wanted revenge on her. As a parting gift, he gave her the sword (and a new set of armor),Brienne chooses the name "Oathkeeper" for the sword.

Oathkeeper remained her Brienne’s possession even after her task was complete.