Sirens Call (One Piece)

"I've seen several great pirates rise and fall during all those years, hell I've even seen people conquering the sea with creatures they used to call dragons. But believe me, if I tell you Luffy, that every single one of them would have never managed that on their own."
*****
While traveling towards Sabaody, the Straw Hats stop at an inconspicuous island. While further investigating the small island, they discover that the lord of the island keeps young girl as a slave. But this certain slave is far more than just a simple human girl.
She is the last siren of Grand Line.

!Careful! May contain a few hints on Pirates of the Caribbean or HTTYD!