In which book does Korra first metalbend?

Korra first metalbends in Book 3: Change. She learns the technique in Zaofu under the instruction of Suyin Beifong, a master metalbender and leader of the Metal Clan.

Metalbending is a specialized form of earthbending developed by Toph Beifong. It works by sensing and manipulating the small amounts of earth impurities still present in refined metal. Zaofu is built around this art: its buildings, domes, vehicles, and security systems all reflect the city's metalbending culture.

Korra's training occurs after she travels to Zaofu while helping recruit and protect new airbenders. Suyin initially challenges Korra to approach metalbending with patience and sensitivity rather than relying only on force. Korra eventually succeeds, making her the first Avatar known to master metalbending. The achievement expands her already unusual skill set: she can bend water, earth, fire, and air, while also using healing, spiritbending, and energybending.

A common confusion is associating Korra's metalbending with Lin Beifong, who is already an expert metalbender and uses metal cables as a police officer. Lin and Suyin are Toph's daughters, while Suyin is Korra's direct teacher.