Oathkeeper (sword)

Oathkeeper is one of two Valyrian steel longswords made from Ice, the greatsword of House Stark, under orders of Lord Tywin Lannister.

Season 4
Tywin gives 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. When Jaime refuses, Tywin lets him keep the sword and tells him to go do his duty.

Following Joffrey's death, Jaime tasks Brienne with finding Sansa Stark and returning her to King's Landing. As a parting gift he gives her the sword, as he knows it was reforged from Ice, so she can protect Ned Stark's daughter with Ned Stark's own blade. She considers a name for a few moments, before choosing "Oathkeeper".

In the books
In the A Song of Ice and Fire novels, the sword is forged by Tobho Mott from Eddard Stark's greatsword, Ice, along with the smaller blade, Widow's Wail.

The sword has black and red ripples through the steel. Its scabbard glitters gold and is decorated with a row of lion's heads and smoldering red rubies. The pommel has a golden lion's head with ruby eyes that shine like two red stars.

Jaime is angry when the sword is given to him, thinking that it is a cruel jape of his father and uncle. Kevan is upset to hear how Jaime feels about the sword, saying the gift was heartfelt, but Jaime is not mollified.

After returning to King's Landing (post PW), Jaime and Brienne are stopped by Ser Loras Tyrell, who believes that Brienne was responsible for Renly's death. Jaime orders Brienne to be locked up for her own protection, thus when Jaime gives her sword she is in a tower cell, not the White Sword Tower. He also suggests the name "Oathkeeper" to her, claiming it is appropriate that she will be defending Ned Stark's daughter with Ned Stark's own steel.