She was a priestess of Auri-El without a name, which means she would have to spend her entire life in The Chantry of Auri-El, that was her destiny, but she wanted to know the world and be free.
Her backstory resembles Van Hohenheim and the Dwarf in the Flask from FMA:B, but she represents  both characters at the same time, because she was trapped in her little bit of world and she wanted to be free to create her own destiny. The role of the Dwarf is fulfilled by Hermaeus Mora who made a deal with her for some secret writings of the Temple of Auri-El that Xarxes himself din't included when he created the Oghma Infinium and she was able to traslate, Mora's help makes her to become a great priestess of Auri-El with the freedom to travel throughout the entire region as will, he also give her a name and magic knowledge, at this point Mora was patient since she was revealing the content of the scriptures of Xarxes little by little.
During the war between the Snelfs and the Atmorans, Mora created a plan to supposedly save the Snelf race, giving them the magic power to defeat the Atmorans, but he was actually testing the scriptures of Xarxes.
Since she was great priestess of Auri-El she had the power of influence in the Snelf King decisions, with Mora's help a lot of battles during the conflict between the Self and Atmorans took place in specific places in order to create a big summoning circle supposedly to channelize the energy of the Et'Ada.
This ends with all the Snelfs accidentally bounding part of their souls with her in a ritual, she becomes immortal *thanks* to this, basically she is now a living sort of black soul gem, most of her race loses their will to live after this, they weren't Falmers *yet* but with their souls consumed by the ritual they were only empty shells of their former selves so they were easily enslaved by the Dwemers. Something similar happened in FMA:B when Van Hohenheim becomes inmortal but in his case everyone's dies.
Anyways, those were not her intentions, she really thought that her race would be saved with that power, but Mora only tricked her and use her and her race as test subjects.
She didn't even know that was Mora's plan but in a twisted way her wish of being free is accomplish, since that day she gained an eternity to freely roam around Nirn with the guilty of what she indirectly did.
The only Snelfs not affected by this where those who refuse participate in Mora's plan for example Gelebor, Vyrthur, the Snow Prince who had arrived into Solstheim.
Also this is my headcanon explanation of why Falmers have lesser souls.