You become the dark counterpart of a shipgirl, a being born from the resentment of the souls lost at sea. Your rigging transforms into an eldritch shape that meshes organic elements with black metal.
You can choose any Abyssal princess that matches your tonnage but not installations. This grants your the ability to summon subordinate vessels through a dark ritual. You can have only have a single strike group of 3 heavy ships and 7 light ships, 2 support ships, and 3 submarines, summoned at once.