The Tale of the Lady and the Lion is a commonly recited myth across the Seven Kingdoms involving Lady Stoneheart (Catelyn Stark) and the Kingslayer, Jaime Lannister. Most know not of how this tale came about, the truth behind it or who was the one who crafted the tale, but most agree that the sudden disappearance of the Kingslayer in 301 AC was discerning.
The tale makes mention of the Brotherhood without Banners and how they came to be lead by what some would call a witch and others a monster. She was called Lady Stoneheart by many and was associated with the death and murder of many in the Riverlands, especially from House Frey. The tale tells how in The Riverlands many were stricken by fear by this group of brigands and that they sought to right the wrongs committed by House Frey during the Red Wedding and anyone who betrayed Robb Stark and brought about his downfall.
This particular aspect of the tale refers to Lady Stoneheart's interactions with Ser Jaime Lannister whom if the tale is to be believed came into contact with the Kingslayer just upon the onset of Winter. The Tale tells that Lady Stoneheart also had dealings with Lady Brienne of Tarth whom had her prized Valyrian Steel blade taken from her.
It is unclear what happened between the Lady and the Lion although the tale suggests that the Kingslayer was captured by Stoneheart and then placed upon trial. Accused of breaking his oath to Catelyn Stark, the Kingslayer was forced into a duel with Lady Brienne of Tarth and one of the pair was slain.
It is unclear who was slain, although the tale continues and says that a knight in golden armor joined the Brotherhood that day and joined in the fight against the Freys, ultimately assisting in restoring House Tully to their place in Riverrun.