What i do now?

[Ever]

I have just beaten Aster and Eclipse at azurine island. Where i can find Cain now?

[Phlytosion]

There's a hidden door in that room. They mention it after you beat them. Don't scroll too fast through dialogue in this game it WILL come back to bite you in the ass later. But yeah, just make contact with the back wall in that room where you beat them and a doorway should open up.