The Sea of Ice

Ever since Katara got her memory back, she's hated Zuko. And who wouldn't? After all, he lied to her and used her, or so she thinks...
But now a new element has arrived, in the form of Twila, a girl from a place that shouldn't exist: a sea of ice between the poles. She says her people are descended from the ancient Spirits, before the time of benders and humanity. But what does she want with Katara and Zuko? What is this Prophecy she speaks of? And who will survive, when the world is at stake?