Completely speculation, inspired by and therefore spoilery through 309 of the manga.
Tale of Madara
Once upon a time there was a little girl named Madara, with hair as dark as night, and skin as pale as parchment, and eyes that became red as blood whenever she wished them to. She was a beautiful girl, but she was also ambitious. And so, when two men came to her family and asked them to help form a village, hidden in the depth of Fire Country, to be a place to raise and train ninja, she counseled her mother and father most strongly to agree. Even the news that the cousins who had cursed them, the hated Hyuuga, were to be part of the village too, failed to sway her, for she knew that true power was to be found in the shadows. And because she was so earnest, yet so gentle, her parents gave in to her entreaties, and the agreed to help establish the village of the Hidden Leaf.
As the years passed, she grew more and more beautiful, and suitors gathered to try and win her hand. But she would have none of them, for her eyes saw not beauty but power, and she was repulsed not by ugliness but by ambition, for her own ambition had grown with her, and would brook no other. Instead, she secretly took her own brother, himself bedazzled by her beauty, as her lover.
Night after night, in her heart of hearts, Madara prayed for cunning to ensnare, the flexibility to entwine, the strength to crush her opponents, and the venom to destroy them. And because this was truly her wish, the eight-headed, eight-tailed demon snake Orochi heard her, and decided to grant her it… after a fashion.
Seven times, Madara's menses stopped and her body began to swell with child, and seven times a presence would visit her after the third month and she would writhe in pain as the child's essence was sucked away, and her blood flowed hot and heavy. The eighth time, as her belly rounded, her brother withered, until she gave birth to a child with the eyes of a snake and skin as soft and white as its underbelly.
She named him Orochimaru, after the demon coiled within him, and used the afterbirth to tie it to his soul. The gift had been given to her son, but she was determined to make it hers anyway.
Because his eyes would never gain the red necessary to his mother's clan, and because he had no father (for his mother's brother/lover lay dead and unacknowledged), the Uchiha name was forbidden him, and he was known only as Orochimaru. But he was his mother's child through and through, and as she grew old and powerful, he attained his majority, cold and spiteful, with the cunning to ensnare, the flexibility to entwine, the strength to crush his opponents, and the venom to destroy them; with hair as dark and long as night, and all the beauty and grace of a snow maiden; with a thirst for knowledge, for life unquenched, and no freedom. For his mother held him close and used him to gain her ends, and that gave him the most potent weapon of all, the snake's last gift: hatred of his own, and desperation to drive him to the one for whom he was named.
The Orochi and the Orochimaru worked together, learned together, studied together, experimented together, planned together, until the day Orochimaru could learn no more from the Hidden Leaf, and could no more bear the weight and chains that bound him to his mother. And so he revealed his madness, reveled in it, and used the discovered experiments as his excuse to flee the village.
And Madara searched for another bijuu to pervert and conform to her own ends. She was an Elder now, her black hair white with age, and her face lined and cracked as an old riverbed where once the springtime flowers of her youth and beauty had bloomed. But she was powerful, for she knew that power lay in shadows, and the hated Hyuuga, the cousins that had cursed her family, were as birds in cages compared to her. But it was not enough.
It was Madara who found the nine-tailed fox, the Kyuubi, whose every step was fire and destruction. It was Madara who lured it to the village, and convinced the elders to try for it without the Yondaime's knowledge or permission. But Madara had worked with snakes for too long, and she misjudged the fox whose every step was bold and dispelled the shadows she knew so well. Because the fox would not stoop to her level, he came directly to kill her, but because he did not seek destruction, he came in human form. On his way, he met no one but a boy five days shy of five years of age.
"Who are you?" the boy asked.
"I am the fox with fire in his step," the Kyuubi replied.
"Why are you here?" the boy asked again.
"I seek Uchiha Madara, who intends to use me against my will."
The boy thought about that for a moment, then said, "Come. I will lead you to her."
The fox followed the boy to Madara, and there he killed her, and left the rest of her family to plot a summoning that would take eighteen weeks and two days to complete, so that they might have their revenge upon the Kyuubi and overthrow the Yondaime as they thought Madara had wanted.
Seven weeks later, there was born a little boy named Sasuke, with hair as dark as night, and skin as pale as parchment, and eyes that would become as red as blood in time, and when he opened them for the first time, Madara saw the light of day again.
.: ende :.
.: forty-nine days till rebirth :.
