Jason Lannister

Lord Jason Lannister is an unseen character in Game of Thrones. He was the Lord of Casterly Rock during the Dance of the Dragons.

Biography
When Dalton Greyjoy and his Ironborn attacked the Westerlands, they were able to burn the Lannister fleet and sack Lannisport. There, they stole grain, gold and hundreds of women and girls, including Lord Jason's favorite mistress.

Reference

 * "The Old Way"