During the Andal Invasion of the Vale, the sword was in the possession of the Andal lord Qyle Corbray. When Corbray was killed by the First Men King Robar II Royce, the latter claimed the sword and used it during the Battle of the Seven Stars to kill a knight he wrongly believed to be Ser Artys Arryn.
In the books
In the A Song of Ice and Fire novels, Lady Forlorn is currently in the possession of Ser Lyn Corbray, the eldest brother of Lord Lyonel Corbray, having inherited it for taking it in the Battle of the Trident after their father, then the Lord Corbray, had been wounded and using it to slay Prince Lewyn Martell. Lyonel still resents not having inherited the blade even though he inherited all family titles and lands.
The Corbrays claim that Ser Jaime Corbray slew Robar in the Battle of the Seven Stars, allowing them to reclaim the original Lady Forlorn.