In the mid first Era of Tamriel, there walked two mortal Nedes - brother and a sister - the last of their tribe. In the wake of their tribe's destruction they wandered. They hired themselves out and grew rich off the treasures they stole from ancient ruins. They lived a harsh, but comfortable life.
Until the reality of their mortal existence caught up with them.
The sister, poisoned by an enemy, grew ill. The brother carried her across the Jeral mountains, searching for anyone that could cure the taint spreading in her blood. Hundreds of leagues, days...weeks walking, carrying the last of his mortal family in his arms. As she grew closer to death, the breath rattled in her chest and eyes grew dull. The brother felt despair, but refused to lose this last link to his lost tribe - he would not walk this life alone.
Near the edges of Cyrodiil, she came: The Other. She stopped the brother on the road, eyeing the sister's pale face, watching her struggle to gasp a breath. Naturally protective, he prepared to defend his sister but when he tried to strike, The Other was too fast, too agile, too empowered. She possessed raw power and learned skill the likes of which he'd never seen.
She made him an offer - I can save her. I can ensure that neither of you lose your last connection to your mortal life and give you the power to avenge your tribe.
The brother questioned the Creature. The Creature would only say that his sister would live and that they, together, would be more powerful than they could imagine. The sister, choking in a breath accepted the gift. He watched as she died and awoke - whole and strong. Eyes wide with wonder, the brother offered himself to the Creature as well.
The Creature stayed with them, teaching them how to embrace the Gift of immortality and how to test the bounds of their power. With their increased power they felt they were destined for something more, and something great required a great name.
The Siblings, Valitore and Theresa, lived eternally, just as they were promised. They grew in power, feeding on the blood of mortals, mesmerizing those that had use and killing those that stood in their way. Using their abilities, they cheated, charmed and seduced their way into more and more power. They achieved nobility and wealth.
But this was not enough.
As the second Era began, they realized that the Empire they'd infiltrated was flawed and weak. Its leaders, mortal emperor after mortal emperor, was lesser for their mortality and short sided view.
They would change the world.
They would rule Tamriel. Slowly, as the empire collapsed, they gathered their allies, fellow nobles and merchants who shared their views and were willing to follow. Those that rose against them were ground under their greater numbers, skill and gold. Invictus stood with two dozen noble houses of commendable strength and vigor, each accepting the gift that the Siblings shared. Their blood was spread. From then on they were Mother and Father to the nobles they turned and the houses that followed them.
There were many hardships - spies infiltrated their ranks many times, killing valued servants. Blood children were assassinated by enemies who saw their immortality as sin. Some rebelled, growing to believe themselves better suited for the throne, only to be crushed under the Siblings power. Houses rose and fell - several dozen at the height, ebbing and flowing with the politics of Invictus and the wider world.
Turning their eyes to the Ruby Throne, power and influence growing exponentially as the Pact, Covenant and Dominion waged war upon each other - the Siblings stood with five great houses. Though, in the search for power, peace, and balance in Tamriel, even that would change...