The Realm of Darkness. A place of total despair. Home to the nastiest types of Pureblood Heartless, and dark beings beyond anyone's twisted imagination.
A small number of such beings, thirteen to be exact, march through the Dark Realm. Strutting arrogantly, almost as if they own the entire realm, which they probably do. They have figures that would make anyone, man or woman, have perverted thoughts just from looking at them. These figures are accentuated by the organic bodysuits they they wear. Together they would be considered even more attractive than the seven Princesses of Heart, were it not for the fact that they were all quite frightening with their glowing amber eyes and wicked smiles.
Yes, these maidens aren't maidens at all, but immortal creatures born from darkness. They whose hearts were made from the shattered remnants of the χ-blade, of the thirteen shards of darkness. They are the true thirteen of darkness, just how the Princesses of Heart are the true seven of light. They are those with hearts of pure darkness, not even the faintest glimmer of light shines within them. Who are they? They are the thirteen Daughters of Darkness. They are so terrible that all in the Dark Realm fear them, even those without hearts.
Their names are branded onto their backs in a language that only they can understand. Their only loyalties lie with themselves and each other, not caring in the least about what happens to others, unless it either harms or benefits them. Their strongest has even gained her own Keyblade, for dark Keyblades are gained when the previous wielder is slain by the current one.
They seek to find a way into the Realm of Light, so that they might enjoy steeping humanity in sin and darkness, corrupting all hearts until they become as dark as their own, extinguishing the seven lights in the process. Then, they will find a way to the Heart of All Worlds, to gain ultimate power and knowledge, becoming the thirteen Dark Queens.
But this is what they believe will happen when they reach the Realm of Light, for the Keybearers of Light shall stand in their path, bringing them down and banishing them back to the Realm of Darkness from whence they came. So rejoice! For these Guardians shall never fail to protect both the Princesses and us in our dire hour of need against the thirteen Daughters.
This is just one of my two theories of what happened to the thirteen shards of darkness that Master Yen Sid mentioned when he was explaining what happened to the χ-blade. My other theory is that they turned into dark crystals that hold the power to become substitute hearts and bring back beings of darkness such as Vanitas. Oh yeah! I don't own Kingdom Hearts, for it belongs to Tetsuya Nomura of Square Enix.