How do you aquire this ship?

The only way to get this ship is by going through the storyline and getting to the very end. You fight it before going into the last level, along with several other ships. But you have to fight pretty hard to take it, its got a large crew and like, 5 or 6 floors if memory serves.

I captured it at the battle near Khael Roa island.
After capturing Greenford to free Clement Aurientius, fighting off the English attack, you have to talk to Clement and sail to Khael Roa island to find the huge treasure there.
But there are 4 ship block you from the landing area: 1 English Manowar and 3 pirates. I catupred the Manowar first, then sank the pirates.
To do that I had: Battlship with full crew member, Danielle and Lucas (level 10, melee 10 and have all fighting attributes) and I reached level 14 with melee 10.
If you have an officef with sailing skill you can remain your old ship by ordering him as captain.