[corporatejay]

I haven't read the books but this is from Wiki but Lady Stoneheart better not exist. That would be absurd.

In an attempt to save her son's life, Catelyn takes Aegon Frey hostage and kills him when Roose Bolton kills Robb regardless, but she still has her throat slashed by Raymund Frey. Three days later, Catelyn is resurrected by Lord Beric Dondarrion, who sacrifices his life force to revive her. However, the period of time she spent deceased has caused Catelyn's body to decay; furthermore, upon her reanimation, she loses most of her previous personality, except for her hatred of House Frey. Catelyn then assumes command of Dondarrion's outlaw band, the Brotherhood Without Banners, and changes their aim to exterminating House Frey. Her uncompromising brutality earns her the moniker "Lady Stoneheart".
A Feast for Crows[edit]
Stoneheart and the Brotherhood come across a small party led by Brienne, who informs Stoneheart that she is searching for Sansa at Jaime Lannister's request. Stoneheart names Brienne a traitor because she carries Oathkeeper, a Lannister sword that was forged from Ned Stark's Valyrian steel blade, Ice. Brienne swears that she is still faithful, but Stoneheart insists she must prove it by killing Jaime, whom she believes played a role in the Red Wedding. Brienne refuses, even when threatened with a hanging. Just before Brienne is to be hanged, she shouts out a word.