After finishing the Energy Temple and Kelp Forest and then exploring the Abyss, someone suggested I check out Mithalas next... that gave me Energy Form, which in turn got me into the Veil, which then gave me Li... so now I have access to the Sunken City (via Li) and the Frozen Veil.
Problem: Light-up Form appears to be required to progress in the Sunken City. I say "appears to be" because it's possible I'm missing something; the little boy runs into the darkness, and there aren't any glowy fish around that I can lead down there with my songs. Is there a way around this problem for those of us who are doing Sunken City but don't have Light-up form yet? Or am I off to Frozen Veil next?