The Twins

Full Item Description
The Twins are blades, and yet they are not. That is, they ARE swords, or rather a single sword split into two, but they are also people in and of their own right. The sword has two parts. There is one half made of of a black crystal, and is a girl named Xiphi. The other half is made of what looks like the purest silver, but far stronger, and his name is Acer. The hilt of the blade is black inlaid with silver, so that the colors show about evenly, a melding of the two, symbolizing their unity.
History
The Twin's history starts with a wizard, as so many magical items do. The wizard's name was Geolain, and he started life as the son of a famous weaponsmith in a poor town. Geolain was never like other children, he was reclusive and never went out to play, he spent his days locked in his room reading books of strange tales, of war and valor and evil - but mostly what he found in those books was a growing insanity. His mental state held enough for him to go into tutelage under a wizard not far away, who recognized the boy's promise in the arts. He was disappointed however - Geolain's insanity showed itself more and more often, and the wizard decided he could not afford giving his teachings to one so rapidly approaching the edge. Geolain, naturally, took his ejection badly, but he went to work for his father, working on swords. It became apparent, however, that he had little talent for traditional smithing - a fact he covered up by using what little magic he had learned.
Over time, he became a renowned smith of the finest quality weapons, though he always worked alone and in secret, never showing how it was he crafted such marvels. It was soon, though, that news of this reached the ears of his old tutor, who caught on immediately. Geolain, however, had 'foreseen' this, in his paranoia, and caught the mage unawares as he came into town. He had discovered on his own the means to enchant the blades, and so one of his enchanted weapons took the life of the one who had expelled him. Before anyone knew what was happening, Geolain gathered his enchanted weapons and rushed to the older wizard's tower, where he holed himself up and spent the next twenty years reading the wizarding books, teaching himself the craft - for he was not just insane, but also brilliant.
He did little in the ways of a wizard, however. While he had all the knowledge he needed, he had very little training, and since word of his assassination of the elder wizard was widespread, none would take him but those he himself considered 'unscrupulous'. Actually he considered everyone but himself 'Unscrupulous', despite his own wrongdoing. So he went back to smithing, making enchanted weapons that he would sell to the occasional purveyor that came once word of his brilliantly crafted weapons got out. Of course, these were all quite seedy individuals, but Geolain decided it was necessary - he needed money for his new obsession - the perfect sword. A weapon that would be superior to anything and everything, something that would prove to everyone he was NOT crazy, but a genius of unparalleled caliber.
His endeavor took him seven years of working in secret, of buying the most expensive and powerful ingredients, of the most potent spells he could summon, and finally, his greatest work - the greatest work of all history, to his mind - was complete. Hammered magically and normally from two exactly opposite materials, both unbreakable by known means, and imbued with his most powerful enchantments, his creation was complete. He delivered the final stroke from his hammer, and pulled the blade from the magical forge he had created himself, which alone had taken two years, then submerged it in the one liquid - also his own creation - that could cool and finish his weapon. It was not to be as he had thought, however. As soon as the blade touched the liquid, there was a tremendous release of energy that destroyed the basin, the forge, and very nearly Geolain himself - but what brought the mage to his knees was what befell his beloved sword - the weapon had sundered. It now lay in two parts on the floor, split into the black and the silver halves, the hilt had divided but was still whole on each blade. He moved to take them, to forge them together any way he could, but before he had the chance, the blades glowed, and began to change.
As the blinding light faded, Geolain found himself staring not at the dual blades he had seen a moment before, but at two small children - twins. They were shaped like humans, but with absolutely colorless skin and snow white hair. The boy, who was to be named Acer, had blue eyes that almost glowed, and the girl, Xiphi, had green eyes that shone in the same manner. The flabbergasted wizard stared at the two for a long time as they lay on the ground, no more that two or three, holding one another tightly. Finally, he walked over and announced that he was their father, the great wizard Geolain, and that he had created them from fire and magic, that they were his children. They both smiled then, silently, and accepted this, and so he took them and raised them as his own children, as promised.
With the twins to care for, Geolain seemed to regain his sanity. He still had the same irrationalities, but he kept them in check, for he had, in a way, accomplished his life's work - they WERE the blade he sought to create. As the years went on, however, and Xiphi and Acer grew, he became increasingly frustrated. They defied all his efforts to classify - they grew like they were alive, yet they had no discernable interior, their blood was like liquid metal, and their skin, which was soft to the touch at sometimes and hard like steel at others, healed over their cuts rapidly. In fact, even when he cut Acer open, not even thinking that it might kill the boy, he seemed to be made entirely of the silvery substance, while Xiphi's insides were purely black. So he felt that they might be golems, yet they had wills of their own. Of course they also did not behave like children, they had little interest in games, and often they simply sat in silence together - they were never apart. What infuriated him the most, however, was that despite the fact that they were obviously magical in nature - their very existence proved it - yet neither exhibited any sign of magical power at all, that even his most sensitive methods could detect.
Geolain kept himself in check, however - they were growing, after all, like children, perhaps their powers were not mature. When they reached their eighteenth 'birthday', however, the mad wizard could take no more. They showed no interest in any sort of combat, nor any sort of magic, only in gazing out at the landscape he never allowed them to visit and being with one another. So, that evening, as they stood on a balcony as they so often did, he went to his lab, and retrieved the scraps of the metals he used to create them. There he forged a dagger, using only minimal enchantments, and went up to them. They both smiled, and greeted him as their father as they always did, thinking nothing of the sharp object he held. He was, after all, their creator. As the moon rose, the wizard bade Xiphi to face him, and he took one last look into her brilliant green eyes - before his plunged the weapon into her chest.
The dagger worked as it was intended. The wound did not heal over, in fact it continued to grow, she began to dissolve from the point where he stabbed her. She screamed out in agony, and Acer clasped her hand, screaming in just as much pain, and still more misery as his sister died in front of him. Her body glowed, and reduced itself down to her core, then there was a flash, like the day they were born, and one half of the wondrous sword, black as the darkest night, with a black and silver handle, dropped to the ground. Geolain let out a yell of triumph, and moved to do the same to Acer, but his creation had been awakened, and in the worst possible way. Moving with a speed the old wizard could never have matched, even in his youth, Acer grabbed his sister off the floor, and sliced his creator's torso from his legs, silver tears running down his face even as a spray of hot blood hit him, and dripped from the edge of the blade he held. She spoke to him then, trapped now in her original form, and they agreed to leave. No one knows what became of the sword sister and her brother, but it is said he roams the lands now, his sister serving as his only companion and his defense, searching for some way, any way, to return her to her body, so that they might be as they once were.
Magic Properties
Since no one has ever wielded either of the blades, their magical properties are largely unknown, and almost everything here falls into the realm of speculation. It is known, however, that the blades are unbreakable as long as the other remains intact - that is, if one makes an attempt, both must be broken at the same instant. Apart from that, almost nothing is known. It is said, however, that were one other than one of the Twins themselves to wield the blade, it would change their very bodies, granting power not through the sword itself, but transferring their power directly from their spirits into the spirit of the holder. The legend holds, however, that the black blade brings power from the demonic, the dark energies of the world, while the silver blade confers upon the holder the exact opposite powers - powers of nature, of the divine. The blades, and so the Twins, are polar opposites in power as they are in color, yin and yang, dark and light, day and night. Though it is said that the two can never be reunited fully, were someone to do so, the power it would create would be more than was ever meant for this world, or any other...

EDIT: Well, first I tried to put stats up here. Apparently those are bad, and I can see why, but honestly I really dont know how to do this so that people won't just tell me 'it has potential'. Which, dont get me wrong, I appreciate that, but... I can't seem to get beyond the 'potential' thing and into 'thats actually good'. SO here is yet another attempt to realize this potential:

As characters or NPCs:
As I said, both have extremely hard skin, almost like armor, and heal fairly rapidly. They are also faster and stronger than an oridnary person, and Acer tends to be rather rash and emotional in his decisions, but this is usually tempered by Xiphi, who is very calm and calculating. Both are also extemely intelligent. They have some magical ability, corresponding to their powers. Acer can wield raw and light energy very well in its pure form, whereas Xiphi can command the opposite. Also, they both have an alternate form that it somewhat more fitting to their nature. Acer becomes a sort of silver knight, with two tremendous angel wings, and Xiphi can become a black winged demon.

As blades: As swords they retain most of the same powers they had as people, but rather than having them on their own they confer these abilities on the user. Acer can heal the user, whereas Xiphi can drain life from those she attacks. Acer's blade is charged with light energy while Xiphi's is charged with dark. Both blades are exceptionally sharp, and will make the wielder both stronger and faster, not quite as much as they are as people, but at least more so than the user would be normally. In addition, Acer can give some of his changing ability to the one that holds him, granting them gleaming silver armor and greatly increased strength. Conversely, Xiphi's blade can warp the user into a kind of demon, similar to her alternate form, but again, somewhat weaker. Essentially they grant the wielder all of their powers, but at a lesser scale.

As blades combined: This is where the real power comes from. Anyone who somehow manages to recombine Acer and Xiphi (no simply holding them together does not do the trick) will gain an incredible increase in strength and power, and the ability to wield almost any element through the blade. Their skin wil be as the toughest metal, and their stamina will be unmatched. In addition, they can use the full power of the blade to change theselves into a being of unimaginable power, the true nature of the blade conferred unto them. A titan wielding a massive blade, with power to strike down any foe (Open to interpretation ^_^)

EDIT (again): I think I'm going to go with Iain's idea regarding their forms, while Xiphi's body was destroyed, she can 'trade places' with Acer on occasion
Author's Note
Suggestions are welcome, as always. I'm thinking about the magical powers a bit more, though I would also like to leave some of that to your imaginations, if I may...
Hope you enjoyed the story

EDIT: Well, thats about all I can think to put. Thanks to Cheka for reminding me that sticking 'Enhancement Bonus +whatever, true seeing, etc.' was BAD. Not a mistake I want to make.