Spoilers:
You might need spirit form to get into the frozen veil; as I recall, you do. However, you will not get sun form by completing the Abyss in the dark, and you can't complete the Abyss without the spirit form to get to the whale and health upgrade (and an easter egg), which you get by completing Mithalas (along with beast form which gets Naija up any current), and sun form is in the temple in the Veil, along with Li (in the Veil, not the temple), whom you need to get into the sunken city and to defeat the Creator. So while we are all impressed that you got to the transit turtle without sun form, you are really doing a lot of stuff with one hand tied behind your back. But you will love the frozen veil; it is just magical, with dolphins, whales, otters, gently falling snow and great music....one of my favorite maps...and you can defeat the mini-boss mantis shrimp there with the forms you have. However, I found him much tougher than King Jelly...bring lots of ingredients and recipes for both. Just because Mithalas has five battles doesn't mean it is tougher than the Abyss, but I think you may miss things like getting memories (for the final ending) wandering around there in the dark...oh, double-click on the mini-map for the world map. There is a lot of help in the gameplay forum, and you can do a search for the threads you need; there is a great Adobe reader set of maps from Momruoy with icons on them of what is there, currents marked, and so on.