I believe I know which one you are talking about. Now here is how I got it: Back when I wasn't subscribing (a few days ago) I helped my friend defeat Foulgaze, after that he got a quest to defeat Lord Nightshade and I helped him with that, once we defeated Lord Nightshade we BOTH got the Savior of Wizard city badge. If you have defeated Lord Nightshade but did NOT get that badge then there must be some sort of glitch.
Jasmine StormBlood Lv. 11 Initiate Sorcerer
A wizard's life is not about magic, magic is about a wizard's life. Life is his body, Fire is the gleam in his eye, Ice is the calmness in his voice, Myth is his creative mind, Storm is his endless energy, Death comes in at the end of a wizard's life but Death also makes sure that Life does it's job again.
Oh I did not know that you had to do the sunken city one! Thanks!
For the Badge you have to do sunken city, but, as far as, unlocking the next zone, I do not think its a requirement. I think every zone has something like Sunken City Instance.