last island
 

there is an island to the southwest wich i have not unlocked, i have already closed the portal to demonis, how do i unlock that last island?

They are four separate islands in fact and if you have just closed the portal to Demonis from Greenwort than you're still in the first half of the game. Don't worry, you'll unlock those islands much later in the game and you'll be able to return to Demonis and Hades as well.