Once, thousands of years ago, there ruled a great and wise Pharaoh with a cunning younger brother. But though the Pharaoh ruled justly and well, and his people worshipped him as a god, other kingdoms sought to invade and damage the peace the Pharaoh so fervently wished to uphold in his kingdom.
His brother and the royal magicians claimed to have found a method of forging great weapons that could fend off the invaders. The king, trusting his brother, agreed to the weapons' creation.
He probably should have read the fine print.
These Millennium Items, as the tome in which their creation was detailed called them, would be capable of a great many things. Each would contribute to the judging of wicked mens' souls, represented in the form of monsters or ka living alongside the person's soul next to their ba, or personal spirit. Each would allow the user to manifest their ka to do battle for them - and would allow them to summon a special alternate-dimension space, known as the Dark Game, where others could summon their ka to fight as well. And each, as a last-ditch method to protect the user, had one final ability: if someone who loved the user were to die protecting them, that martyr's ka would be sealed within the Item and the user could fuse their ba with it, enabling them to physically transform into a half-ka creature with tremendous magical power. The Items responded to love.
But they also required sacrifice. Just to create them, 99 human sacrifices were required to fuel the creation of the magical gold. The Pharaoh's brother targeted the tomb-robber village of Kul Elna, seeking to both create the Items and eliminate an internal threat to the kingdom in one fell swoop. No one expected there to be any survivors. Yet during the raid on Kul Elna, a woman fell defending her young daughter from attack, and the daughter escaped unscathed save for the horrible memories etched in her mind.
The woman's ka, in the meantime, inhabited one of the resulting Items from the very beginning, making the "Millennium Ring" a dark, unstable artifact that only the strongest of the Pharaoh's magicians could control. With the Items' assistance, the Pharaoh and his priests successfully defended the kingdom and judged all criminals who dared defy the just rule of their king. The daughter, meanwhile, grew up nursing a hatred of the Pharaoh on whose decree her family and friends were murdered. Her ka fed on this hatred and the souls of those she defeated over time, becoming a mighty and influential monster to serve a mighty and influential thief.
By the time this daughter, Thief Queen Bakura, had reached adulthood, the Pharaoh's own young daughter Atem had claimed the throne after the Pharaoh learned the truth of the Millennium Items and died of grief. The Thief Queen blamed the daughter for the sins of the father and declared an all-out war against the royals, seeking to collect for herself the 7 Millennium Items her village had died to create and lay them to rest. The first Item she reclaimed was the Ring, whose bearer Mahad's ka sealed itself in the Millennium Puzzle around Atem's neck as a result of the strong bond between magician and master. The Ring made Bakura stronger, and something about it felt familiar. She learned to transform.
The war waged on. The Pharaoh's priest Set (secretly her cousin) rescued, fell in love with, and lost a young woman named Kisara whose ka sealed itself in Set's Millennium Rod. Yet one by one all the Items fell into Bakura's hands, and she placed them in the Tablet of the Pharaoh's memories - only to unleash a dark god, Zorc, upon the world. To seal Zorc away again, the Pharaoh sealed her own spirit in the Millennium Puzzle, erasing her own memories and decreeing that all mention of her be removed from the royal records, so no one would ever remember what could happen or try to free Zorc again (despite prophecies that this would happen anyway). Zorc, however, melded itself with the Thief Queen's soul and took up residence in the Millennium Ring, waiting millennia for the Pharaoh to return to the world, killing all who tried on the Ring until a suitable host could be found. She wouldn't be strong enough to summon her ka right away, but that was all right. She had methods of gathering souls to feed it. It was all only a matter of time.