Crescent Island

I’ve heared that there exist a second island. When can i see it? I have best tiamat.

Crescent Island is the only island to go on for now. (Well, and the whole ice area which is pretty much an island. And the entire main area, which is also an island. But that’s it).

I dont understand that. How i get to crescent island. 

Keep playing the main story

If you already defeated Tiamat, you already finished the game and should be on Crescent Island. Crescent Island is the last place you visit.