Disclaimer: I don't own the X-men franchise or anything related to it. All I own is Adanya and this plot.
About 50,000 years ago…
A tribe of mutants with powerful abilities holds claim to the area surrounding them, offering protection and a home to any mutants powerful enough to work with them and killing anyone who isn't. It is from this initial tribe of mutants that all others are descended. Watered down over the years by human blood, many born without any manifestation of the powers their ancestors, only a few in whom the x-gene becomes apparent and gifts them with powers, far weaker than their ancestors, but descended from them nonetheless.
It is to this original tribe that a child is born, a young girl with more power than any other. And the moment her mother, holding the child swathed in furs in her arms, brushed her lips against her head, the woman sagged, death taking her in an instant. The girl is named Adanya.
Adanya's brother returns home from a hunt gravely injured. The three year old Adanya cries and hugs him and he is healed.
At four, another child in the tribe burns Adanya with his powers. She heals instantly and the fire child drops dead at her look.
At five, she watches on as her father is brutally murdered by men raiding the village. Every attacker falls dead in the next moment. The next morning as the attackers are burned and the dead are being prepared for burial, they suddenly wake. The soil in the tribe's graveyard begins to move, and every dead village member rises. Many have been dead for years. All this so a little girl does not lose her remaining parent. When she next goes to sleep all those brought back fall dead again.
The villagers decide that Adanya is too dangerous, her powers are out of control and the risk of her killing them all in a fit of temper, or once again raising the dead, regardless of how desiccated they were. So they find a practitioner of old magic and banish the girl into a rainforest, the magic trapping her there for thousands of years.
And the world went on. The mutants spread out and began breeding with humans, the legendary stories of their strength and power lost to time. And the girl watched, through the gaps of her beautiful cage, as her family forgot she ever existed.