He is captured by the forces of House Stark. Robb Stark interrogates him and learns that he has estimated the size of the Stark army as 20,000 men. Robb feigns mercy and lets him go in an effort to trick Tywin Lannister into thinking 20,000 Northmen were marching against his army, when in actuality Robb had only sent 2,000 men. This by extension has Robb defeat Jaime Lannister's army at the Whispering Wood, with Jaime being captured himself.