After over seven years of waiting, Avatar fans finally found out what happened to Zuko's mother, Ursa.
We first learned about her in the second season episode, "Zuko Alone" (also the only episode Aang, Katara, and Sokka do not appear in). It that episode we learn that Ursa left the kingdom after presumably helping kill the current Fire Lord because he ordered that Zuko's father kill Zuko. Ursa leaves the kingdom and we never learned what happened to her.
Zuko is my favorite character from the series so I was happy he finally got closure on where she went. I did find it surprising, since we saw how devoted she was to her children how it had happened and I'm curious if to see where the story goes from here.