As you will see at the end of the 3rd book, there is a type of skill-stone that absorbs the memories and essences of skill users. A coterie of skill users go to where Verity went and sculpt dragons from the stone and are absorbed into the stone once they finish it, bringing the stone dragon to life. These are the dragons of legend. Well, if a person touches the stone long enough but not to the point they are absorbed, they are left in nothing but a shell, no memories and no essence of life. This is what the pirates do to people they forge. They have a block of this stone in one of the ships. The ironic thing is that the skill dragons were created to combat the pirates, but the dragons who flew over the pirates forged them on accident by force of their beings. So the pirates are seeking revenge by forging Fitz's people (forgot the name of the kingdom) which in turn will lead to unleashing the dragons again...you see the vicious circle.
Oh, and the pirates have a block of that stone thanks to a character you will meet in the 3rd series