The Legend of Korra, Season 2

Unalaq:
The spirits are angry because he's here. Haven't you ever wondered how your father ended up in the South Pole? Why he's never taken you to visit his homeland in the North?

Tonraq:
Unalaq, this is not the time.

Unalaq:
You're right. You should have told her a long time ago.

Korra:
Told me what?

Tonraq:
I left the North pole because... I was banished.

Korra:
You were banished from the North? Why?

Tonraq:
Because I almost destroyed the entire tribe. Twenty years ago, I was a general in the Northern water tribe, sworn to protect my people.

Young Unalaq:
Brother, we're being attacked.

Tonraq:
I drove them out of the city and deep into the frozen tundra. We tracked them deep into an ancient forest. Many believed this forest was the home to spirits, and the barbarians retreated there because they thought wouldn't attack on such hallowed grounds. They thought wrong. We captured the barbarians, but in the process, we destroyed the forest. I didn't realize the consequences of what I had done. By destroying the forest, I unknowingly let loose angry spirits. They threatened to destroy everything. The entire city. Unalaq was able to guide the spirits back to the forest, but by then, the damage had been done. For being the cause of so much devastation, my father banished me from the Northern water tribe in shame. That's when I came to the South and started a new life.

Bolin:
Whoa. So you were supposed to be chief. Then he became chief. No wonder you guys don't like each other.